Für den Fall der Fälle (sprich wenn Ihr nicht weiterkommt)
Leider in englisch aber besser als keine:
1) Create a copy of your single player desktop shortcut
2) Right-click on it and select "properties"
3) In the "Target" field, add the following to the very end (outside the quotes): +set sv_cheats 1
4) Click OK Launch the game using that shortcut
Once you're in the game, you should be able to use the following cheat codes (hit the "~" key to bring down the console, then type them in):
/god (makes you invulnerable)
/notarget (makes enemies not attack you)
/noclip (lets you walk through walls)
/give all (gives you all weaps and ammo