At the pause screen press:
     Up, Up, Down, Down, Left, Right, Left, Right, B, A.

The typing console will appear then type in the cheat:

Cheat code:             Effect:
MorgyTheMole            First Person/Headless
upthejoltcola           Faster Music
chipmunks               Squeaky Voices
stevebrooks             Infinite Secondary Items
patrickdugan            Animal Sounds
ArieannDeFazioWithFlair Increase the Size of Jennifer's Chest
reverb                  Remove Film Grain
SouthPeak               Push Objects Away from Player
ihatefunkycameras       Disable special tracking cameras


Achievements
     Complete each achievement to get the allotted gamerscore.

"It's Mine! ALL MINE!" (25): 
     Collect 10,000 Monster Tokens across all characters.

A Little Help Goes A Long Way (20): 
     Grab  some buddies and see how many waves you can wade through in
the Co-op Dojo!

Archaeologist (45): 
     Collect 72 Specialty Melee Weapons!

Are You Crazy?! (10): 
     Frag 4 players in under half a second in any ranked match!

Are You Related To A Belmont? (20): 
     Don't  just  hide. Fight back! Score 30 monster stuns in a single
ranked monster hunter match!

Control Freak (20): 
     Perform all flag captures in a ranked CTF match!

Demolitions Expert (20): 
     Score all frags using only grenades in a single FFA Deathmatch game!

Did Someone Order Chinese? (15): 
     Find  a  way  to deal with that pesky Granny evil cat shield! (on
Thriller or higher)

Dirty Deeds (10): 
     Score  10  frags  by  summoning homing explosive zombies while in
zombie form in a single ranked match!

Does Anyone Clean This Roof? (15): 
     Collect all weapon parts on Hell School Rooftops. (on Thriller or
higher)

Exhumed (15): 
     Smash all the gravestones in the hidden graveyard and survive the
horror! (on Thriller or higher)

Fashion Show (70): 
     Unlock every character costume.

Go, Speed Boater, GO! (15): 
     Zeppelin  Flyer  X  and The Leprechaun has challenged you to beat
his record! (Thriller or higher)

Grumpy Old Man (15): 
     He's  old  and grumpy, so try to show him you care by not letting
him get hurt. (Thriller or higher)

Have Fun Cleaning The Grill! (10): 
     Take road rage to the extreme and frag 20 players while driving a
vehicle!

High School Flag Team Captain (20): 
     Become  a  flag capping beast by using the flag to kill 4 players
in a single ranked CTF match!

Honed Survival Instincts (10): 
     See if you can get far enough going solo in the Co-op Dojo!

I Said Turn Off Your Car Alarm! (15): 
     Destroy  all  those pesky, loud cars in the Parking Lot with your
mech! (on Thriller or higher)

I Think We're Being Followed... (15): 
     Bring  out the white knuckled driver in you and blast through the
escape! (Thriller or higher)

JAILBREAK! (15): 
     There  has  to  be  something  useful  in the castle jailcells...
right? (Thriller or higher)

Junkyards Just Aren't The Same (15): 
     Make a mad dash to Bob Zombie! (on Thriller or higher)

Killing Machine! (20): 
     Get 50% of the total kills in a ranked FFA Deathmatch game!

Leading Noobs To The Slaughter (20): 
     Destroy  the  opposing  team by having your team win with 100% of
the kills in a ranked Team Deathmatch

Long, Hard Road Out Of Suburbia (20): 
     Travel 1,000 kilometers

Maniac! (30): 
     Kill 10,000 monsters.

Master Of Your Domain (20): 
     Enter and never leave the hill in a ranked King of the Hill match!

Monster Survival Guide Pays Off! (10): 
     Try  to  win  a  10  minute  or longer ranked Monster Hunter game
without being turned in a monster!

Monsterologist (20): 
     Unlock every Bestiary entry.

Nice Guys Finish Last (20): 
     Become  a  saint  and win a ranked friendly fire token race match
without killing any other players!

Prowler On The Roof (15): 
     Collect  all weapon parts in Streets of Suburbia. (on Thriller or
higher)

The Bodyguard (15): 
     These  gold zombies trust you with their life. Show them you mean
business. (on Thriller or higher)

They Wouldn't Spend It Properly (15): 
     Let  the  monsters know that you're looking after their best gold
interests. (Thriller or higher)

Trap Assassin (15): 
     In  Zack's  House,  use your cunning to defeat all of the enemies
with traps! (on Thriller or higher)

Very Sneaky, Sir (15): 
     Show  them  you're  a  ninja of the night when taking back what's
rightfully yours. (Thriller or higher)

We Don't Need No Stinkin' Ammo! (15): 
     Resupplying  is  for  the  weak!  Beat the final showdown without
buying more ammo! (Thriller or higher)

What's That Sound? (15): 
     Why  waste  ammo  when a massive, angry ball of death will do the
work for you? (Thriller or higher)

When Household Appliances Attack (15): 
     Show  the  monster  masses  that vacuuming can be fun... by using
excessive force! (Thriller or higher)

You Got Fur In The Hyperishdrive (15): 
     Can  you escape the garbage trucks of death without boosting? (on
Thriller or higher)

Your Kung Fu Is Strong (20): 
     Defeat  all  challengers using only your melee weapon in a ranked
FFA Deathmatch!