First, find the hammer that is supporting all the junk. When Fry picks it up, he will die and the screen will say 'Game Over'. However, this is only a joke. After the Professor explains about 'The Re-animator', he will give Fry the hammer. Go over to the lockers. Open the locker that reads 'Hermes' and you will see a red cage with a Nibbler in it. Smash the cage with the hammer, and touch the Nibbler. Once a Nibbler disappear, it indicates that you have rescued it. You can only rescue the one in Hermes's locker after you get the hammer, because you have to smash the cage open