The reason the addChild didn't work is because your class wasn't a display type, not to mention you're doing it all wrong. If you want a class with code and the image of MovieClip from the library. Right click your MC in the library and click 'Linkage'. Then click export for actionscript and put the class name as Star. Then your code would look something like this.
// Document class
package
{
import flash.display.MovieClip;
public class DocClass extends MovieClip
{
function DocClass()
{
var star : Star = new Star();
addChild(star);
addEventListener(Event.ENTER_FRAME, enterframe);
}
function enterframe(e : Event) : void
{
star.move();
}
}
}
// Star.as
package
{
import flash.display.MovieClip;
public class Star extends MovieClip
{
function Star()
{
x = y = 50;
}
function move() : void
{
x ++;
y ++;
}
}
}
This should, haven't tested, put an instance of your star on the stage at (50, 50) and slowly move it towards the bottom right. Hopefully you can see what I'm doing. Rather than having a class with the code in which then controls a class in your library you do it all in one class.