The nook is found in Chapter 2 (Optical).
In the room where you have to form the giant light green chess piece, you'll notice that there's a door half way up the wall on the left of the room (from where you form the chess piece). Use the chess piece and the checkerboard cube to form stairs up to that room, and go to the end. Don't forget to use the fire alarm next to the door and the fire extinguisher in the nook.