var el:MovieClip = new MovieClip;

el.addEventListener(Event.ADDED_TO_STAGE,center);

private function center(evt:Event):void{
	evt.target.x = (stage.stageWidth - evt.target.width)/2;
	evt.target.y = (stage.stageHeight - evt.target.height)/2;
}

just a basic center function

Center a left-top corner based element in stage

ActionScript 3