Adding tracks to the server is simple.
1. Save a .trackinfo file for the track you want to upload to the server. You can do this by starting a single player race on the track and typing "/savetrackinfo" in the chat. This will save the .trackinfo file in your MX Simulator personal folder.
2. Once you have this file, navigate to the "Tracks" tab of your control panel.
3. Simply drag and drop one or more trackinfo files into the upload box.
4. Now click the "+" button at the top right of the track list.
5. Finally, select a track and add an amount of minutes and laps.
Below is a video showing an overview of the control panel. It is slightly outdated, but tracks are still added in the same way. The tutorial for adding tracks starts at 5:25.