stage.stageHeight/Width

[Vlykarye]

This post is most certainly improved.

The first problem is that you cannot access the stage through "stage." because that returns a null value.

[Vlykarye]

You should just pass the stage variable into the class through the constructor.

Then, you should rename the functions "onKeyDown" and "onKeyUp" (which are reserved function calls for AS2) to "onKeyPress" and "onKeyRelease" to get rid of any compiler warnings.