First, make sure that you collect all 64 cubes. Once you have done that, head to the Star Gate in the 32-cube room for a new ending. Once the credits roll, select New Game+ again from the main menu. Play until you reach the point where previously you received the fez and the sunglasses. This time, you'll receive a pair of stereoscopic 3D glasses, which automatically enable Stereoscopic Mode. You can disable the effect in the Settings menu, if you wish.