How do you unlock motor cycles in Burnout Paradise for Xbox360

Make sure you have the latest version: There are some instances where you are not notified that a new version is available on 360. If you are unsure simply go to the Xbox Live Marketplace, select the Game Add-ons section and verify that the Burnout Paradise 1.8 item is ticked. If it isn't please be sure to download it from the Marketplace to get updated.