Hmm.. not sure, where is it that you are receiving the error? (i.e. is it on the DPS website, or are you getting past the bit where you put your credit card details in and then getting the error when returning to your own website?)
I haven't heard of the pxaccess kit, so didn't install this. I only installed 5 files: pxaccess.inc, ps_dpsax_notify.php, ps_dpsax.php, ps_dpsax_config.php and checkout.dpsax_result.php which I got in a zip file somewhere further back along in this forum.
If all else fails you could try emailing the DPS team, and they were very helpful with my problems!