I couldn't really find another place to ask this but I hope this is ok. I'm trying to play realism (3.0) mod with a couple of friends, I was wondering if it's possible to play multiplayer with friends, we tried it today and I'm not sure if it's a problem with us or if multiplayer isn't possible.

If you load the coop menu you can select the map in the list if you scroll down, you dont want to set the game to zombies but select it from the coop-map list ( when you have the mod loaded obv ).
Then you should be able to play with friends just like any other zombie map..