Go to "Main Menu" > "Extras" > "Bonus Content", then then enter one of the following codes to activate the corresponding cheat:

CHEAT FUNCTION - CHEAT CODE
Urban Combat single player level - sp-street
Village Assault single player level - sp-village

Assault rifle
Large ammo clips, good rate of fire, optional grenade launcher attachment. Use on light armored vehicles and infantry.

Light machine gun
Large ammo clips, high power, highest rate of fire. Deadly to infantry and most vehicles. Use with troopers on the frontline.

Sniper rifle
Small ammo clips, low rate of fire, highest accuracy. Use for one-shot kills.

Rocket launcher
Low rate of fire, high power, lock on feature. Use on vehicles.

Shotgun
Medium ammo clips, high power. Use in small areas.

Silenced SMG
High rate of fire, high power, C4 attachment, extremely quiet. Use for stealth missions.

Press ~ during game play to display the console window, then activate the following cheats by entering the corresponding codes:

CHEAT FUNCTION - CHEAT CODE
God mode - god
All weapons and ammo - loaded
Maximum ammo - allammo
No clipping mode - ghost
Disable AI - playersonly
Slow motion - slomo <number>
Walk mode - walk
Flight mode - fly
All weapons - allweapons
Third person view - behindview 1
Kill player - suicide

ACHIEVEMENT - HOW TO UNLOCK
Anvil (15 points) - Complete the third mission on normal difficulty.
Captains of Industry (15 points) - Complete the second mission on normal difficulty.
Darkness Falls (15 points) - Complete the first mission on normal difficulty.
Graveyard (15 points) - Complete the fourth mission on normal difficulty.
History Repeats (15 points) - Complete the final mission on normal difficulty.
Living Quarter (15 points) - Complete the sixth mission on normal difficulty.
The Mountain King (15 points) - Complete the fifth mission on normal difficulty.
Anvil - Ironman (20 points) - Complete the third mission in a single session without dying.
Darkness Falls - Ironman (20 points) - Complete the first mission in a single session without dying.
Graveyard - Ironman (20 points) - Complete the fourth mission in a single session without dying.
History Repeats - Ironman (20 points) - Complete the final mission in a single session without dying.
Living Quarter - Ironman (20 points) - Complete the sixth mission in a single session without dying.
Anvil - Stopwatch (25 points) - Complete the third mission in under 25 minutes.

Start the game with the -devmode command line parameter, then press ~ during game play to display the console window. Enter "god" to activate God mode.