I haven't tested this and it's not the most elegant, but I'm pretty sure it'll "work":

/admin/index.php around line 48 you'll see this segment. Add the line in red:

Code:
<?php require(DIR_WS_INCLUDES . 'header.php'); ?>
<!-- header_eof //-->
 <?php
if (zen_is_superuser() == false) die();


  $customers = $db->Execute("select count(*) as count from " . TABLE_CUSTOMERS);