You have to buy the game to download latest updates. just in case you have bought it, click updater.exe in your beamNG main folder.
Have you bought the Alpha? Or are you using the Techdemo? You can't have multiple cars in the Demo, let alone AI However, if you have the Alpha then go to this Wiki Page: http://wiki.beamng.com/Enabling_Ai_after_recent_patch PS - I just finished improving the wiki article
If you have bought alpha, please check your email again, and see the link with it. click on it and get promoted on forums, that will make you able to see private threads.
You might want to upgrade you're account if you bought it. There should be two emails that were sent to you. Also to update your game go to start, all programs, BeamNG-drive, and click on BeamNG drive update.
I have the same problem, there are no updates available past 0.3.01. The version.txt file in the main directory contains: 213
The AI is still infact broken. It rarely works correctly and even if you do switch to thier vehicle and back to yours, and even if you get that to work, switching back to your vehicle may disable ai again. The system that runs the AI needs to be fixed and its by pure luck and miracle to get it working.