Marketpress gateway custom plugin for Paypal Advanced

I have recently purchased the MP plugin. I upgraded from the lite version for the sole purpose of adding this plugin to a clients site and hooking it up to their Paypal payments advanced account. I know that you don't have that option, but I have been modifying an existing gateway plugin and I have reached a small impass. Everything seems ok (it always seems ok right?), but the response from paypal is always that "RESULT=1&RESPMSG=User authentication failed" in the testing environment. Can I possibly get some help with my code, and if so, where do I need to send the snippets? I am a newbie on this, however I do understand a bit of php. Thanks in advance!