If you've completed the game and cannot find the Beetle in the car selections, then it's not there.
If you have a cheat device (Gameshark or other cartridge device, or Xport and other USB device) then you can use these to place GT3 cars into the places of the GTC cars, including the Beetle 2.0, Beetle RSi and Beetle Cup car.
Why you'd want to is another matter entirely.
