Free Play 
Successfully complete all of the training missions (every icon has three things under it) to unlock free play for arcade mode.

Get Shotgun Shells
Go on special mode and get 1500g (silver in boxies, plant pots eg) then go shopping. Choose weapons and pick the shotgun shells and then buy. Then go back and pick equip shot gun shells. Whenever you go on special mode you can use a shot gun(shot gun shoots slower but shoots off about 5 bullets and only uses 1 shell).

Game Shark Codes