Start the game and type any one of those cheats (without "") when playing the game.

Code           Result
---------------------
"MONEYMONEY" - get 1000$
"HITMENOW"   - Play as HITMAN
"JOBDONE"    - Current mission completed 
"MAXPAYNE"   - Play as MAXPAYNE
"LIONARE"    - Always get 100% of health
"POLICECAR"  - Spawns a POLICE CAR