<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/02/03/displaying-a-main-icon-in-a-flex-popupbutton-control/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="top"
        backgroundColor="white" viewSourceURL="srcview/index.html">

    <mx:Style>
        PopUpButton {
            popUpStyleName: myCustomPopUpStyleName;
        }

        .myCustomPopUpStyleName {
            fontWeight: normal;
            textAlign: left;
        }
    </mx:Style>

    <mx:Script>
        <![CDATA[
            import mx.controls.Alert;
            import mx.controls.Menu;

            [Bindable]
            [Embed("assets/asterisk_orange.png")]
            private var asteriskOrangeIcon:Class;

            [Bindable]
            private var menu:Menu;

            private function popUpButton_initialize():void {
                menu = new Menu();
                menu.dataProvider = arr;
            }
        ]]>
    </mx:Script>

    <mx:Array id="arr">
        <mx:Object label="Alert" />
        <mx:Object label="Button" />
        <mx:Object label="ButtonBar" />
        <mx:Object label="CheckBox" />
        <mx:Object label="ColorPicker" />
        <mx:Object label="ComboBox" />
    </mx:Array>

    <mx:PopUpButton id="popUpButton"
            label="Please select an item"
            icon="{asteriskOrangeIcon}"
            popUp="{menu}"
            initialize="popUpButton_initialize();" />

</mx:Application>