Here is some info on how to add tracks ... and where to find them.
Together with some friends I have compiled what we call a basic mp set ... let's say some tracks which are used a lot (and loved) and where to find them.
To add a track:
add to the racer folder in data/tracks
Tip: racer does not seem to check if you have the correct track when you join a mp game. If you have weird results when joining a game - double check you have the correct track.