Had a couple of problems installing this payment system but got there in the end. Here's a list Q & A's I wanted to share for future reference:
 
Q. Unknown URL error.
A. In EPDQ cpi admin tool ' Allowed URL' use: http://www.example.com/payment.php
 
Q. Post URL error.
A. If http://www.example.com/payments/epqd_cpi_response.php does not work try http://hosting.viart.com/~example.com/payments/epdq_cpi_response.php

how do I setup the "POST User name" and "POST Password" sections the payments folder by default are password protected so what do I enter into these 2 fields??