Forum:Undead Nightmare not Showing Up! PLEASE HELP

I just downloaded Undead Nightmare for PS3. It seems to be fully downloaded and installed. However, when I go to the main menu of RDR I click on Single Player, Undead Nightmare and it brings me to the DLC menu. Undead Nightmare isn't there, and it says to enter a rockstar code. Where do I get that, if I need it? I downloaded the 20$ version for the Playstation Store. If ANYONE could help, it would be appreciated. I've searched for hours for a solution. Thanks.