(flex4): addChild() is not available in this class.
flex4中：
just create another container wich u place displayObject in:
// container ( IVisualElement ) for DisplayObjects
var container:UIComponent = new UIComponent();
addElement( container );
// displayObject goes to container
var displayO:Sprite = new Sprite();
container.addChild( displayO );
?
flex3中：
var container:UIComponent = new UIComponent();
addChild( container );