Using this Trainer:
Launch the trainer first, then launch the Game and then Press F1 at the Main Menu.
Listen for "Activated".
Press desired option key
Numpad 1: Unlimited Team Health
Numpad 2: No Reload
Numpad 3: Unlimited Ammo
Numpad 4: Super Jump
Numpad 5: Petrified Zombies
Numpad 1: Unlimited Team Health - you and your team are protected.
Numpad 2: No Reload - your current weapon will not need to reload. Ignore the falling bullet count, it will loop to 99.
Numpad 3: Unlimited Ammo - you have 999 bullets for current weapon. Pistols already have unlimited ammo.
Numpad 4: Super Jump - you have more powerful jump
Numpad 5: Petrified Zombies - zombies are rooted and cannot move. They can still attack if you get close. You can shoot them and then press the key to allow them to die. May not work with all Zombies.
Unlock more trainer options including updates from here - Left 4 Dead 2 Trainer