At the Mission computer press ctrl+F1 . Then You can enter one or more of the following codes. All the codes are case sensitive !!! Correct codes will be confirmed by a beep.

Here are the codes:

Redtop Trod - Select all single player missions from the mission Menu.
Half Libel - Activates Teleport key.
Had A Nude On - Activates Invincibility key.
Brass Clue - Activates Amunition Key.
Mother Mourn Us - Changes mountain on mission 1-5 to Mt. Rushmore with programers faces.
Chaste Coed - View the end video sequences.
Rotted Drop - Activates Destroy Target.
Horny Elk Leer - Activates all targets on radar key.
O'Sarge - Gives Seargeant in training missions an Irish accent.
Swiss Throat - Changes Citadel building into 7th level headquarters.
A Mere Fart - ???
A Scramble On - Let's you take screen shots by pressing SHIFT-CTRL-RIGHT CLICK.
Range Goes Gory - All Levels, and lots of other stuff (I don't know exactly what it does)
Dunk It Here - Recording Session Outtakes (I don't know exactly what it does)
Oh No! Less Japan - Activates Ion-Strike Key
In game Control+X for Ion-Strike

In-Game-Keys : CTRL+I - Activates Invinciblity
CTRL+Z - Refills ammo
CTRL+F - Destroy current target
CTRL+B - Teleport Behind Target
CTRL+P - All targets on radar