To create a new menu item on an existing menu such as the "Customers" menu, there are 3 simple steps:
1. Create a new file in the extra_boxes folder, perhaps like this:
/admin/includes/boxes/extra_boxes/myorderbrowser_customers_dhtml.php
and put the following inside it:
Code:
<?php
$za_contents[] = array('text' => MYORDERBROWSER_LINK_TEXT, 'link' => zen_href_link(FILENAME_MYORDERBROWSER, '', 'NONSSL'));
2. Then create a language file for the link name:
/admin/includes/languages/english/extra_definitions/myorderbrowser_definitions.php
Code:
<?php
define('MYORDERBROWSER_LINK_TEXT', 'My Order Browser');
3. And define the filename in a new file as well:
/admin/includes/extra_datafiles/myorderbrowser_filenames.php
Code:
<?php
define('FILENAMES_MYORDERBROWSER', 'myorderbrowser.php');
There are lots of examples of admin addons that have added extra menu items. One such example is the "Backup MySQL Plugin". You could use it for inspiration or as an example of the pattern to follow.