You need to click the link included in the email that contained your license key to upgrade your forum account. If the email you used to purchase the game and register your forum account are different: http://www.beamng.com/threads/1471-...ed-Invalid-serial-Problems-on-Account-upgrade
There should be a link in the email you got that changes your profile to say that you own Drive. (So when you click it, you should get the Drive logo like mine) EDIT: Ya beat me to it.