When you start free ride extreme, go to the most north eastern point of Oakwood. Now place yourself so that you're looking down the street. Look at your right and you should see a telephone booth (on the sidewalk). There is a house with a blue door just right of the telephone booth. Go up to the door and the action icon will appear. Click the right mouse button (DEFAULT Right Mouse Button) and the icon will disappear. Nothing will say that you've unlocked all Free Ride Xtreme cars, but if you check the Free Ride car selection screen, the cars should be listed there.