<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
        layout="absolute" initialize="openModalWindow()">
 
    <mx:Script>
        <![CDATA[
            import mx.containers.Canvas;
            import mx.managers.PopUpManager;
            import mx.containers.TitleWindow;
            import mx.controls.Label;
 
            public function openModalWindow():void
            {
 
                 //define title window
                var _titleWindow:TitleWindow = new TitleWindow();
	        _titleWindow.title = "New modal window";
 
	        //define and add canvas
	        var _tmpCanvas:Canvas = new Canvas();
	        _tmpCanvas.height = 100;
	        _tmpCanvas.width = 300;
	        _titleWindow.addChild(_tmpCanvas);
 
	        //define and add label
	        var _tmpLabel:Label = new Label();
	        _tmpLabel.text = "Your modal window content.";
	       _tmpCanvas.addChild(_tmpLabel); 
 
	        //add modal window
	        PopUpManager.addPopUp(_titleWindow, this,true);
	        //center modal window
	        PopUpManager.centerPopUp(_titleWindow); 
           }
	]]>
    </mx:Script>
 
</mx:Application>

PopUpManager.addPopUp(_titleWindow, this,true);