Here is a quick tutorial on how to ensure you are not using a beta version of rF2. First load Steam and click on the rFactor2 label and then choose Properties. the next screen will display several tabs. Choose BETAS. When you click the Betas tag this screen will appear. You can use this screen to backtrack to many older versions of rF2, but to get all the new DLC content working you need the latest version of rF2 so select the NONE option. To be safe, exit and then restart Steam. Then when you launch rF2, you'll be running the latest available version which should be compatible with all the new DLC cars and tracks.
