My solution: enter the PSN-Store, open your download-list (right top) download the pass again. For me it worked, I hope it does work for you also...
I forgot to answer your questions, EvangM.
Were these sealed, unopened copies of the game? Yes
Did you enter the code from the back of the manual? Yes
Did you double-check that you did not perform a typo? Yes
It is writes that "The code you entered may not be correct or may no longer be valid."
Here's my fix to this issue -
I made a new user account on my PS, then I set up a new PSN ID with the PSN store that was native to my region (in my case it was EU and incidentally, native to the disc as well). Logged in through that, redeemed my codes, logged out, logged back in through my US PSN and everything works fine. Hope this helps others.
It just sucks that games are region free but DLCs are not.
I just tried what you wrote down. Created a new psn, an EU PSN, then I log in to my another US PSN and it's still didnt work.. I want to play multiplayer so much...
btw, i never had this type of problem, that i paid 40£ for a game and i just cant activate it.
I finished the single player in 4-5 hours on normal, which was great, but it was really short.
Then I cant play multiplayer, because this dumb code dont want to work...
And still waiting...
if you followed those steps and it still didn't work, i guess look around for another solution to it