When I try to play co-op in Saints Row: The Third, it says I need to enter a code. However, I have already done this and got my unlockable items like the car, gun and clothing. I have been trying to sort it out, but nothing has worked, can you help?
Sounds you like you got a pre-order bonus, and you've entered the code for that. Did you enter a separate code for an Online Pass? It's possible and pretty likely that they are 2 completely different codes on different pieces of paper in the game box.
I ordered mine after the pre-order bonuses and the game only came with a card with the Online Pass code. The online pass card should look something like this: