Wallace & Gromit: Project Zoo Cheats

After the fight past the Octave Security Gate, after the elevator, you
will see a round platform. Push down the TNT box. Fight the bad guys.
After you beat them, go into the doorway you opened. Go through it and
get the zoo token. Keep going farther into the tunnel. Go through a
sewer pipe all the way to the end and you will appear at the bonus
level!