At the menu with the "Campaign", "Bonus", and "Options" selections, press Down, Right, Left, Up, Down(2), Left to unlock Generation 1 Cartoon Optimus Prime. If you entered the code correctly, the screen will flash. Select the "Bonus Features" option, then choose the "Generation 1" selection to use the new appearance. Alternately, collect all the Autobot secret icons in the Autobot campaign.