This occurs in Chapter 3, and you must first turn the power on and make your way to the tent with the keyboard. It's the same tent as the Slideshow.
Once you're at the Keyboard, play these notes
1) 5th White Key
2) 2nd White Key
3) The Last Black Key
4) The Last White Key
5) 6th White Key
Your character will hum a tune to let you know if you've done it right