I downloaded this free version from GOTD and liked it so much that I purchased it. They sent the reg code but I have tried several times to register the purchased version and it tells me that my activation code is invalid. Any suggestions?

I would like to... suggest an idea I downloaded this free version from GOTD and liked it so much that I purchased it. They sent the reg code but I have tried several times to register the purchased version and it tells me that my activation code is invalid. Any suggestions?