MarketPress Not Sending Order to Authorize.net

HELP!!! I'm lost on why my orders won't go through using MarketPress and Authorize.net and it's costing me money daily!!! I really don't want to have to start over with another product.

Problem:
When I try to order a product from my store, it goes through the process ok, but after confirmation, when you choose to CONFIRM PAYMENT it thinks then brings you back to the CONFIRM PAYMENT page and displays a message in red:

There was a problem finalizing your purchase. Please go back and try again.

TRY AGAIN is a hyperlink that takes you back to the PAYMENT INFORMATION page.

It's not my card as it never gets to Authorize.net which is set for TEST MODE. I have confirmed that my SSL certificate is activated and it is assigned to an IP address so I know it's not a server issue (that i'm aware of). I've checked the API key from Authorize.net about 100x and what I have entered is correct.

Domain: http://www.hammersgrassfedbeef.com

I'm lost and have to get this fixed. Any help is greatly appreciated!!!

Thanks,

Larry