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.