Start the game with the "-cheats" command line parameter. Then, select the "Cheats" option on the "Special Features" menu or press ~ during the game. Now, you can use the following codes:
Level select: mw_givealllevels [0 or 1]
God mode: godMode [0 or 1]
God mode for team: godModeTeam <#>
No clipping: noclip [0 or 1]
Win mission: mw_winmission [0 or 1]
Lose mission: mw_failmission [0 or 1]
HUD: mw_showhud [0/1/2]
AI: mw_aiDisableThink [0 or 1]