Sorry you are having issues. I honestly do not think that mods or custom content would stop an expansion from showing up. I could be wrong though. It seems like the problem is that your computer is not seeing the game for whatever reason.
I would definitely uninstall and re-install it. I recommend you follow this guide at MTS about how to properly uninstall an expansion. Follow the steps carefully. Hopefully a full clean uninstall/re-install works!