You might be doing it right, but I remember when playing a first party game, it gave me the option of redeeming the online pass while I was still in game(I think Uncharted 3.)
The thing is, in game it didn't work. But when I backed out and went to account management to redeem codes, it worked. If you haven't tried it on the XMB, try it there.
Wait a minute. You have a physical copy? If it's digital I'd assume it works online without any sort of pass.