If you view your registrations, does it show up? Did you log in with the same e-mail account you used when you purchased it? The serial gets attached to your email account, so you need to use the same one to log in.
If neither of those are it, you'll have to contact [email protected] to see what happened with your account. If you still have your order summary email from back then, pulling off the order number or whatever would probably help speed it up.
