How do you open the apartment door on Shrink Ray Island when shrinked?

The cat will only enter C.J.'s apartment once, and has to be chased out. First follow him into the kitchen, then into the bathroom. When you leave the bathroom the cat is gone, and is back outside the apartment. This is because someone has slipped in through the door. When you look into the microscope, you are attacked by the masked intruder.