AS3 Way to AttachMovie

MovieClip:

var ball_mc:BallInLibrary = new BallInLibrary();
addChild(ball_mc);
ball_mc.x=100;
ball_mc.y=100;

Button:

var button_mc:ButtonInLibrary = new ButtonInLibrary()
addChild(button_mc)
button_mc.x=200;
button_mc.y=200;