Check the MP forum (
http://www.shrapnelcommunity.com/thr...&Board=d3maars). The community isn't huge, so you may have to wait a little bit for a game to begin recruiting and I'd suggest sticking to newbie games the first few times. The game host for whatever game you join should be able to tell you whatever you need to do to get setup for that particular game.
Also, there's a DOM3 IRC channel where people meet for faster paced games.