Instructions
Make sure your PS3 is an original 20 GB console, if you did not buy it brand new. As you can replace the hard drive inside the PS3, it might not have come with a 20 GB hard drive originally. If it does not have any silver coloring or WiFi indicator lights on the front, it is a 20 GB model.
Turn on your PS3 console and eject the disc that is already in the console, if necessary. Push your PS2 game disc into the slot-loading disc drive and it should start automatically.
Navigate across to the "Game" icon in your PS3 home screen menu bar, if the PS2 game does not start automatically. Navigate down to the disc icon in the "Game" menu. This will look like a silver disc, instead of the blue disc icon you see when you load a PS3 game.
Press the "X" button on your PS3 control pad to launch the game.