Hello!
First, as you said, you must patch to the latest build which is currently build 49922.
Then, download some TS12-compatible Multiplayer sessions. I set up a filter in Content Manager like this:
Name=MP
OR
Name=Multi
AND
Name=TS12
This shows up just about all the sessions and routes there. If you download the session, the route will download as a dependency. Then go back and get the route's dependencies and any of any other assets in the session.
To start a session, just click on Drive Session as you would on a single-player one. Existing MP sessions will show in the third column of numbers from the right in the Routes menu.
Kieran.