PC

You know that button in the campaign mission settings screen, under "password", that shows whether you want to play a co-op game or a versus game? It's a pretty useless button for me, since I'm not allowed to click it. I'd really like to be able to play the story against humans, because co-op campaign is too easy on Normal and ragequittingly hard on Hard. Is there something I can do to unlock this, or is it just there to taunt me?