While  in  the game, hit the "`" key to bring up the text screen.
Then type any of these codes:

Code                               Result 
give (weapon)                      Give Weapon (See List) 
give (weapon)ammo (ammo amount)    Give Ammo for Weapons (See List) 


     NOTE:  Certian weapons aren't evalible in all levels. To get them
just  type  their  name and drop the word ammo. For the amount of ammo
type 1. Also, most weapons can only hold a max of 200 ammo.

Those weapons are: 

Colt45
Bazooka
Machinegun 

(Example: give bazooka 1) 

Weapons List: 

Machete
Colt45
Rifle
Bazooka
Machinegun
Flamethrower ("gasoline" also works)
Landmine