In a clean and standard TS12 install, there is 1 MP session. You can find that session under the Debrechen route. To play and with that create an active multiplayer game, you simply start that session. If you notice a session with a green dot in front of it, then you have spotted an already active multiplayer session. If you like to join that session simply do the same as with any normal session.
If you get errors trying to join or create a MP game, besure that your have your game registered, downloaded all updates, and eat enough fruit. If you dont like the default installed MP session, either create your own or download others from the DLS. Note that it is a SESSION that makes something MP, not the route, so look for sessions with MP or Multiplayer in it (and ofcourse also download the route and every dependency of both). To create your own MP session, add the "Multiplayer Rule" to your session and upload it to DLS. Please dont upload every crappy session (and route); if it aint good, people dont want to play it and it will still be a solo game.