Go to the factory level and cross the room to get to the door. Enter it and fall down the first shaft you see. Break open the boxes for coins. Then stomp on the purple block that is second from the right. Crawl to the left and stomp the block and pipe in the middle of the area. Keep on raming into the pipes (going left) until you come to a door. Enter it and become "flat." Climb up the platforms and float to the star door.