Hi there
You need to place the following URL into the DPS PSAccess URL box:
https://www.paymentexpress.com/pxpay/pxpay.aspxThen setup a test product with the value of something like $1. Purchase it via DPS with a valid credit card, and you'll be taken to an external page hosted by DPS.
Because you have no MAC key, you will be prompted with an error message - something like "NN". Once you've seen the error code, ring DPS and tell them.
We had this exact same problem when using the module. After ringing them with the error code, they emailed us a Mac and Access key. Never had any problems since.
Anyway i hope this helps.
Mike