Macromedia is giving an example how to use the window component and one function is to attach a movieclip, for example a textfield. However, when you try it out and want to show some text in the textfield it won't work. The reason is that before the movieclip is attached the line to show the text is already executed. A simple way to bypass this is to use an onEnterFrame function as shown below in the example. 
// create a new CSSStyleDeclaration named TitleStyles
// and list it with the global styles list
_global.styles.TitleStyles = new mx.styles.CSSStyleDeclaration();
// customize styles
_global.styles.TitleStyles.color = 0xff0000;
_global.styles.TitleStyles.fontSize = 14;
function popup(wContent:String) {
	myButton.onPress = function() {
	   tw = mx.managers.PopUpManager.createPopUp(this, mx.containers.Window, true,
	    {closeButton:true, titleStyleDeclaration:"TitleStyles"});
	   tw.title = "WINDOW";
	   tw.setSize(120, 60);
	   tw.contentPath = "name";
	   /**************************************
	   //HERE IS THE IMPORTANT PART TO INSERT
	   this.onEnterFrame = function() {
	       if (tw.content != undefined) {
	           delete this.onEnterFrame;
	           tw.content.myName.text = wContent;
	       }
	   };
	   /***************************************
	   tw.move(20, 20);
	   var handleCloseObject:Object = new Object();
	   handleCloseObject.click = function(evt:Object) {
	       evt.target.deletePopUp();
	   };
	   tw.addEventListener("click", handleCloseObject);
	};
}
popup("Textfield from movieclip!");