Hold shift while selecting "New Game." Any individual level may be selected for game play. Also, at the "Choose Campaign" menu, type unlockcheatmode. If it worked, you'll hear a sound.
If this doesn't work, try cheat below:

Type IAMCHEATER at the opening screen or any menu. [NOTE: If you're playing version 1.1. of the game, try typing iwillcheat] Enter the following codes during gameplay and if entered correctly, you will hear a clicking sound.

CANTDIE - god mode
NOHITS - god mode
GOODHEALTH - 100% health
OPENALLDOOR - open all doors
ALLLOOT - all items available
ALLITEMS - all items available
ALLAMMO - all guns and ammo
KILLTHEMALL - kill all enemies
SHOWTHEEND - show end movie
GAMEDONE - complete current mission
GAMEFAIL - fail current mission
RESURRECT - bring team members back to life
ENEMYF - view enemy
FUNNYHEAD - big head mode
ENEMYF - view enemy
DEBUGDRAWWIRE - wire mode
PLAYERCOORDS - shows your position
LARACROFT - alternate uniforms
ZOMBIE - come back as a zombie