In the first level in New Quack City, if you remember pushing a model building to block a camera, there is a computer in the computer room that is destructable. Shoot it until it explodes and you will unlock a secret multi-player level. Note: Your green target will turn red on the described computer. The target will turn red if you can shoot something and get a reaction -- keep that in mind when exploring.
Game Shark Codes
Master Code (Must Be On) EC8D633C 1456E60A
All Weapons 3CBB786B 1456E7A7
Invincible 3CBB786C 1456E7A1
Invincible/TS Camera 3CBB786C 1456E799
Auto Aim 3CBB786C 1456E7C5
Invincible/Auto Aim 3CBB786C 1456E7C1
Invincible/TS Camera/Auto Aim 3CBB786C 1456E7B9
Rocket Camera 3CBB786D 1456E7A6
Distant Camera 3CBB786D 1456E79D
Tiny Characters 3CBB786D 1456E7B5
Rocket/Distant Cameras 3CBB786D 1456E79E
Rocket/Distant Cams/Tiny Characters 3CBB786D 1456E7AE