This.MC returns undefined

I have having trouble pointing to a MovieClip on the stage.
The instance name is ‘ball’.
Inside the MovieClip itself I tried:

trace(this.ball);
this.ball.x = 0;

The trace returns undefined and when the next line is run it says “a term is undefined an has no properties”.

How do I point to an MC on the stage from inside a MC?
Thanks!