Instructions
Follow Gus the Gremlin until you reach the first door just after receiving your paintbrush powers.
Aim your cursor using the Wii Remote. The Wii Remote cursor is the white circle on the screen. Point the controller at the screen and move it to aim.
Aim the cursor just above the door at the transparent gear between the two orange gears. This gear is grayed out to indicate that you can use your paintbrush on it.
Press and hold the "B" button while aiming at the gear with the controller to spray it with paint, causing the gear to become painted and the door to open.