It depends on what build you have. If it's 49922 (or equivalent) or lower, use the Routes menu to find a route with multiplayer sessions - one of the columns is amount of multiplayer sessions. For 57720 (or equivalent) or higher, there should be a Multiplayer button on the main menu screen.
Makes sense. In your case it will be in the Routes screen, but it will not specifically be called Multiplayer - once you've found the column for Multiplayer sessions, click one of them with a number above 0 in that column, then click View Sessions. The multiplayer sessions will appear in the list (along with any single-player sessions) - choose a multiplayer session and click Drive Session.
In a standard TS12 build 49922 install there is only 1 multiplayer session available, which can be found under the Debrechen route.
All other sessions need to be downloaded first before you can play them. If other people are playing them and they are not on a route you have than they will not show up. In other words: You will have to download a lot of sessions (with their routes) before you will ever see any multiplayer activity.
I think it is easier to find some action by hanging around the multiplayer forum and see when some guys are setting up a "meet". They're similar to Events on VATSIM; specified date and times when people get together to uh, fly uh..together.
