Open add_customer.php in a text editor and look for this line..

Code:
<form method="POST" action="/admin/add_customers.php?action=add_complete">
Remove the /admin/ and all should work....

Code:
<form method="POST" action="add_customers.php?action=add_complete">
The file is called from admin and should not have to address itself. I had the same error, and found it due to renaming admin to something else.