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.

The rail rifle is a really nasty weapon if you can fast reload consistently. One thing I don't like about the fast reload, though, is that the timing varies subtly from shot to shot, so you have to ...