In the main menu, there's an EXTRA graffitied in the background. Click that, and then click again once it zooms in, and you'll end up in a bedroom. There's a game console there, click that, and you'll open it.
On the main menu, right behind the "Start" button, is a building with the word "Extra". Click that word twice or so, and you should be in another screen. Click on the gameboy in the room, and enjoy!
