Here's a quick rundown -
Download the full game from this site http://www.mx-bikes.com/?page=downloads You can't run a dedi server and play with just the Steam version.
Once installed (no key needed, running dedi's is free), you'll need to edit the new shortcut created by the installation of the game as per this info - http://forum.mx-bikes.com/index.php?topic=197.0 Now decide the port you'll use (and open this up on your firewall/router) and choose the name for your config file to put in the shortcut as illustrated.
Then install the mods you want to run into the dedi servers bikes and tracks folders (no need for any paints server side).
Lastly create a text file, place it in the dedi server installation folder (the place where the mxbikes.exe file is) and rename it to what you set in the dedi shortcut also making sure the file extension is changed from .txt to .ini. Then select the code from the link above and paste it all into that .ini file. You then edit this text to get the server to run with the settings etc you like.
The game shortcut should now launch a dedi server with your settings. If you get other settings check your file names etc as if they're wrong the server will still run but also fall back to default settings.