Press [ESC], then the Team Chat key (L), then enter any of the cheats after /tell.
EXAMPLE: /tell godmode

Code - Result:
godmode - God Mode
ammo - Guns No Longer Use Ammo
stats - Game Stats
fly - Fly Mode
invis - Invisibility
winmission - Win Mission
losemission - Lose Mission
give # - Give Item # (0-44)
fovs - Enemy's Field of View