Read this if the game doesn't load.
Zombotron 1
When the game loads, you get the option to choose the control as WASD or ESDF. WASD will be selected by default if it is not changed. You are given a certain target in each level that you must meet to clear the level. Aim and Shoot with the mouse.
Related; Zombotron 2, Zombo Tron 2 Time Machine