Instructions
Start the game and select the desired zombie map. Wait for it to load onscreen.
Play the game as you normally would. If you haven't played it before, the objective of the game is to survive as many rounds as you can without being overcome by zombies. Repair the windows, open doors to new areas and pick up weapons as you find them to defend yourself. Eventually, a zombie will drop a power-up or multiple power-ups after you kill it.
Run up to the power-up (the green icon hovering over the ground) to get it. A pop-up message will show onscreen indicating which power-up you got. How to use the power-up will be indicated in the game.