If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
Pick up weapons and power ups as you advance through the city but watch out as you are heading towards where the zombies are coming from, so the further you go, the more zombies that you will encounter on your journey.
Move with WASD.
Left click mouse to shoot.
R for reload.
1-2-3-4-5 choose your weapons.