Does it say it's already registered, or that it's invalid?
In either case, you should contact [email protected]. If it says already registered however, there's not much they can do, since you either bought second-hand, or the store you got it from resold a used copy as new (Gamestop is known to do this and it's likely not the only one). If it says invalid, then they should be able to straighten you out.
