I did IT!! 
First you need not to edit any ../class/order.php file, simply find the lines in /admin/orders.php:
PHP Code:
// 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.ip_address ";
and add the field you need to be searched at the end, for me it is:
PHP Code:
// 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.ip_address, o.ORDER_FLAG ";

p.s. I know i know.. the convention!!! sorry but i can't edit the previous post so i keep it uppercase to be consistent