First off all make sure you sign into gamesas before trying to redeem it. If you have already tried and failed, you can try deleting your cookies, and redeeming it again.
If you continue to have problems please bare with us, we are working hard to resolve this issue right now, and ensure everyone can enjoy Crysis 2 to the fullest as soon as possible. I will post any updates in this thread.
EA FAQ on this topic: http://support.ea.com/app/answers/detail/a_id/4983/
Temporary work around (possible fix)
If you continue to have problems you should try using the "Forgotten password?" link in the gamesas sign in box. Some users have reported this solved their problems.
We will continue to work on a complete fix as soon as possible.