Enter at the menu that has campaign,credits and bonus features listed:

G1 Megatron:
Down,Left,Left,Down,Right,Right,Up.

Infinite Ammo:
Up,Down,Left,Right,Up,Up,Down.

Jazz G1 skin:
Left,Up,Down,Down,Left,Up,Right.

No vehicle's riding on the streed and no tank's/cop's attack:
Right,Left,Right,Left,Right,Left,Right

Robovision Optimus Prime skin:
Down,Down,Up,Up,Right,Right,Right

Starscream G1 skin:
Right,Down,Left,Left,Down,Up,Up.

Unlock All Missions:
Down,Up,Left,Right,Right,Right,Up,Down.

Unlock Bonus Cybertron Bonus Missions:
Right,Up,Up,Down,Right,Left,Left.

You can get G1 Optimus Prime only if you have all autobot icons in all the levels.

Happy cheating :)