For mac, so long as you get all the files in the right place, this should work. Start the game as usual, and go to the Community section, then User Mods up top. Select Alien Swarm and hit Activate.

My heavy weapons guy killed lots of swarm scum, and my hacker opened a door that only he could open. So there. Anyway, I'm definitely up for a romp tomorrow with 3 of you guys. Anti said there was no ...