Reference the stage from a custom class

I have created a custom class that extends the movie clip

on frame 1 of the main timeline I create it

var mainBox2:mainBox = new mainBox();

Now as I understand it you have to cast the parent as a MC so in the constructor of my external class I try this

var myStage:MovieClip = MovieClip(this.parent);
trace (myStage);

I get null. What am I doing wrong?