sample.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
                layout="vertical">

    <mx:Script>
    <![CDATA[
        import mx.containers.TitleWindow;
        import mx.managers.PopUpManager;
    ]]>
    </mx:Script>

    <mx:Button label="ポップアップを開く"
               click="PopUpManager.createPopUp(this, PopupWindow, true)" />
</mx:Application>


PopupWindow.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:TitleWindow xmlns:mx="http://www.adobe.com/2006/mxml"
                  layout="vertical" width="400" height="300"
                   title="ポップアップウィンドウ"
                   close="PopUpManager.removePopUp(this)">
    <mx:Script>
    <![CDATA[
        import mx.containers.TitleWindow;
        import mx.managers.PopUpManager;
    ]]>
    </mx:Script>
    <mx:Label text="ポップアップウィンドウです。" />
    <mx:Button 
            label="Cancel"
            click="PopUpManager.removePopUp(this);"/> 
</mx:TitleWindow>