You have to beat levels by accomplishing a goal and it must be on Realistic or Directors Cut
mode. After you get the cheat, you can find them in the main menu under cheat mode. 

Alien Episode 1
You must bite off ten live civilian heads. Then, you can use a weapon called Sniper Bite. It let's
you bite off heads from a REALLY long distance! 

Alien Episode 3
Fifteen dead civilian head bites. This cheat is very interesting. It's not just stick figures, as you
might believe, but auctual pipe cleaners wrapped around for enemies. 

Marine Episode 1
Beat the level with at least 80% head shots. This gives you slow motion. 

Marine Episode 2
40% accuracy at least. This gives you 99 grenades in your pulse rifle. no bullets, and cannot pick
up any other gun. 

Marine Episode 3
Finish in under four minutes. This gives you warpspeed. 

Marine Episode 5
32 alien kills. All alien enemies start with a missing leg. They crawl around bleeding leaving a trail
behind them. 

Predator Episode 1
Get atleast 80% accuracy. (Use the shoulder cannon on the blue vision mode. It makes it easier.)
You get a mode called pigsticking. It makes the speargun have infinate ammo, and has a shotgun
like spread! 

Predator Episode 4
Must have atleast 40 Xenomorph kills. This cheat will make your enemies made out of flames.