I'm having the same problem on PC. I used to have the 360 version(traded it for PC) and was able to switch between co-op and versus. Can't change it on PC though.
Edit: Found the answer on teh Steam forums. Can't play versus in campaign on PC. Would have been niced if they removed it from the UI instead of just graying it out. Excerpt from interview:
Q: One of the big things you guys are pushing is seamlessly integrating SP, co-op and MP all into the same thing. How exactly does this work on PC, seeing as they have dedicated servers? How can you have a private game, where only your friends are invited, or switch from singleplayer, co-op and MP on a dedicated server?
A: The trick is that we use server sharing to grab idle dedicated servers so that players can jump on as needed to start up their campaign and run their campaign sessions. However, as it turns out, our campaign play is only 90% compatible with dedicated servers, not the full 100%. Basically, PC players will be able to play through the entire story campaign as a solo or co-op experience (up to 8 players in co-op), but to play competitively (i.e. versus), you’ll have to play in “Freeplay” mode, which is where players get a lot more control over how the game works (can set different match settings, can vote on which map to play next, can turn on VOIP or friendly fire, can play stopwatch, etc. etc.).
We debated long and hard here at Splash Damage about what was the right thing to do: let players play through the campaign in versus, or have dedicated servers, and we decided in the end that dedicated servers were the most important thing to PC players. I’m sure your readers will let us know if we made the right choice.