The FatFreeCart.com service is provided for free without any support whatsoever (that's a major reason it's free :^), so it's entirely up to you to get that set up and troubleshoot any problems you may encounter.
However, I can suggest it sounds like you may just need to make sure your button code is complete and correct, especially your Google Merchant ID (you can find this ID in your Google Checkout account's Settings > Integration screen).
