The master servers went offline in 2011. So multiplayer was disabled in the game's main menu and can no longer run. There are currently no plans to update it any further.
In order to play multiplayer, all clients have to revert to the previous version listed below. Then use the commandline option to connect to a player that's hosting a game. You can also use the console server to run your own server. Because the server is a small app, you can run it on the same machine as the client. See the multiplayer docs for more info.
