SIMCITY 4 Registration Question! Please Help??? I've Tried Everything!!!??

I bought Simcity 4 regular version like a week ago from walmart for $10.00 (lol) and I'm just trying to register it now, but it says that the serial number, I type it in but it says its all ready been registered, or is not valid. On the cd disk tray, there is a product key with numbers bellow it that have numbers and letters. This is the registration code, right? The game came with only the disk, so no box or manual, no nothing! Please help I don't know what to do!!!! Thanks everyone! Oh, and happy holidays!!!