MurderSpot currently uses the Caelestia IRC network for Internet games, matchmaking and general chat. You may use any IRC client to connect. I recommend X-Chat for Windows, and either Irssi or X-Chat for Linux. Note that you can use the MurderSpot program itself as an IRC client - with the default settings, it will connect to the main MurderSpot room.
Use server address irc.murderspot.com and port 6667. Join channel #murderspot for the main MurderSpot chatroom. Remember that you need to use the MurderSpot program if you want to join a game.
You may also chat directly from your browser, using web-chat.