Sorry to bump this post. The first page instructions work great for 1.5.4. I now have working field order_frequent_code populated from checkout_shipping page.
I am trying to echo order_frequent_code in admin/orders.php page, I have tried number of different things but missing something. Maybe someone has the done it already.

Here is my 1.5.4 modified code attempt:

admin/order.php

PHP Code:
  <?php echo $order->fields['frequent_code']; ?> <?php echo zen_date_long($order->info['frequent_code']); ?> <?php echo zen_draw_input_field('frequent_code'); ?> d </td>

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<?php// create search filter  $search = '';  if (isset($_GET['search']) && zen_not_null($_GET['search'])) {    $search_distinct = ' ';    $keywords = zen_db_input(zen_db_prepare_input($_GET['search']));    $search = " and (o.customers_city like '%" . $keywords . "%' or o.customers_postcode like '%" . $keywords . "%' or o.date_purchased like '%" . $keywords . "%' or o.billing_name like '%" . $keywords . "%' or o.billing_company like '%" . $keywords . "%' or o.billing_street_address like '%" . $keywords . "%' or o.delivery_city like '%" . $keywords . "%' or o.delivery_postcode like '%" . $keywords . "%' or o.delivery_name like '%" . $keywords . "%' or o.delivery_company like '%" . $keywords . "%' or o.delivery_street_address like '%" . $keywords . "%' or o.billing_city like '%" . $keywords . "%' or o.billing_postcode like '%" . $keywords . "%' or o.customers_email_address like '%" . $keywords . "%' or o.customers_name like '%" . $keywords . "%' or o.customers_company like '%" . $keywords . "%' or o.customers_street_address  like '%" . $keywords . "%' or o.order_frequent_code  like '%"  . $keywords . "%' or o.customers_telephone like '%" . $keywords . "%' or o.ip_address  like '%" . $keywords . "%')";    $new_table = '';//    $new_fields = ", o.customers_company, o.customers_email_address, o.customers_street_address, o.delivery_company, o.delivery_name, o.delivery_street_address, o.billing_company, o.billing_name, o.billing_street_address, o.payment_module_code, o.shipping_module_code, o.order_frequent_code, o.ip_address ";  }} // eof: search orders or orders_products    $new_fields = ", o.customers_company, o.customers_email_address, o.customers_street_address, o.delivery_company, o.delivery_name, o.delivery_street_address, o.billing_company, o.billing_name, o.billing_street_address, o.payment_module_code, o.shipping_module_code, o.order_frequent_code, o.ip_address ";?>

OR ============================================

<?php    if (isset($_GET['cID'])) {      $cID zen_db_prepare_input($_GET['cID']);      $orders_query_raw =   "select o.orders_id, o.customers_id, o.customers_name, o.payment_method, o.frequent_code, o.shipping_method, o.date_purchased, o.last_modified, o.currency, o.currency_value, s.orders_status_name, ot.text as order_total" .                            $new_fields "                            from (" TABLE_ORDERS_STATUS " s, " .                            TABLE_ORDERS " o " .                            $new_table ")                            left join " TABLE_ORDERS_TOTAL " ot on (o.orders_id = ot.orders_id and ot.class = 'ot_total') " "                            where o.customers_id = '" . (int)$cID "' and o.orders_status = s.orders_status_id and s.language_id = '" . (int)$_SESSION['languages_id'] . "' order by orders_id DESC";
//echo '<BR><BR>I SEE A: ' . $orders_query_raw . '<BR><BR>';
    
} elseif ($_GET['status'] != '') {      $status zen_db_prepare_input($_GET['status']);      $orders_query_raw "select o.orders_id, o.customers_id, o.customers_name, o.payment_method, o.frequent_code, o.shipping_method, o.date_purchased, o.last_modified, o.currency, o.currency_value, s.orders_status_name, ot.text as order_total" .                          $new_fields "                          from (" TABLE_ORDERS_STATUS " s, " .                          TABLE_ORDERS " o " .                          $new_table ")                          left join " TABLE_ORDERS_TOTAL " ot on (o.orders_id = ot.orders_id and ot.class = 'ot_total') " "                          where o.orders_status = s.orders_status_id and s.language_id = '" . (int)$_SESSION['languages_id'] . "' and s.orders_status_id = '" . (int)$status "'  " .                          $search " order by o.orders_id DESC";
//echo '<BR><BR>I SEE B: ' . $orders_query_raw . '<BR><BR>';
    
} else {      $orders_query_raw "select " $search_distinct " o.orders_id, o.customers_id, o.customers_name, o.payment_method, o.frequent_code, o.shipping_method, o.date_purchased, o.last_modified, o.currency, o.currency_value, s.orders_status_name, ot.text as order_total" .                          $new_fields "                          from (" TABLE_ORDERS_STATUS " s, " .                          TABLE_ORDERS " o " .                          $new_table ")                          left join " TABLE_ORDERS_TOTAL " ot on (o.orders_id = ot.orders_id and ot.class = 'ot_total') " "                          where (o.orders_status = s.orders_status_id and s.language_id = '" . (int)$_SESSION['languages_id'] . "')  " .                          $search " order by o.orders_id DESC";
//echo '<BR><BR>I SEE C: ' . $orders_query_raw . '<BR><BR>';
    
}
// Split Page


admin/includes/classes/order.php
line 62
PHP Code:
      $this->info = array('currency' => $order->fields['currency'],                          'currency_value' => $order->fields['currency_value'],                          'payment_method' => $order->fields['payment_method'],                          'payment_module_code' => $order->fields['payment_module_code'],                          'shipping_method' => $order->fields['shipping_method'],                          'shipping_module_code' => $order->fields['shipping_module_code'],                          'coupon_code' => $order->fields['coupon_code'],                          'cc_type' => $order->fields['cc_type'],                          'cc_owner' => $order->fields['cc_owner'],                          'cc_number' => $order->fields['cc_number'],                          'cc_cvv' => $order->fields['cc_cvv'],                          'cc_expires' => $order->fields['cc_expires'],                          'date_purchased' => $order->fields['date_purchased'],                          'orders_status' => $order->fields['orders_status'],                          'total' => $order->fields['order_total'],                          'tax' => $order->fields['order_tax'],                          'last_modified' => $order->fields['last_modified'],                          'ip_address' => $order->fields['ip_address'],                         'order_frequent_code' => $order->fields['frequent_code']); 
line 23

PHP Code:
    function query($order_id) {      global $db;      $order $db->Execute("select cc_cvv, customers_name, customers_company, customers_street_address,                                    customers_suburb, customers_city, customers_postcode, customers_id,                                    customers_state, customers_country, customers_telephone,                                    customers_email_address, customers_address_format_id, delivery_name,                                    delivery_company, delivery_street_address, delivery_suburb,                                    delivery_city, delivery_postcode, delivery_state, delivery_country,                                    delivery_address_format_id, billing_name, billing_company,                                    billing_street_address, billing_suburb, billing_city, billing_postcode,                                    billing_state, billing_country, billing_address_format_id,                                    coupon_code, payment_method, payment_module_code, shipping_method, shipping_module_code,                                    cc_type, cc_owner, cc_number, cc_expires, currency,                                    currency_value, date_purchased, orders_status, last_modified,                                    order_total, order_tax, order_frequent_code, ip_address                             from " TABLE_ORDERS "                             where orders_id = '" . (int)$order_id "'");