Enter the following combinations of icons at the password screen to unlock the corresponding level:

Level 2: Fly, Blue Dinosaur, Gazoo, Snaggletooth
Level 3: Alligator, Alligator, Snake, Gazoo
Level 4: Frog, Fly, Snake, Gazoo
Level 5: Alligator, Snaggletooth, Snaggletooth, Snake
Level 6: Gazoo, Dinosaur, Fly, Crocodile