Yes that was the problem I had as well, I think you have to have bought a game card first when you are going to registrer your account. Don't worry, the game card will not kick in untill after a month, so you will get your free month anyway
If you want, then you can registrer with your credit card, and switch to game cards after account creation. That was how I circumvented(sp?) the problem, by using my brother's card.
I hope this helped