I am using Marketpress Ecommerce and my Payments pathway is not working

I have the free version of marketpress and tried and failed to use framemarket- I did not want it to infect my entire website. I am not sure where to get help with marketpress- this will not allow me specify marketpress as the thing I have a question about.

I am trying to set up paypal as our payment gateway, but even though I have double and triple checked our API signature, it is still giving me an error message when I try to go through the whole checkout process. I have nowhere to go to ask this question, please direct me to where I can actually get the information I need to make your product work!