<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute">
<mx:Script>
<![CDATA[
import mx.collections.ArrayCollection;
private var myData:ArrayCollection = new ArrayCollection([
{label:'a11',url: '../dol/assets/images/lalign.png', side:'left'},
{label:'a323',url: '../dol/assets/images/calign.png', side:'center'},
{label:'asdf45',url: '../dol/assets/images/ralign.png', side:'right'}]);
]]>
</mx:Script>
<mx:List id="myList" dataProvider="{myData}"
width="100%" height="100%"
variableRowHeight="true"
verticalScrollPolicy="auto" horizontalScrollPolicy="auto">
<mx:itemRenderer>
<mx:Component>
<mx:VBox width="100%" height="100%"
verticalScrollPolicy="off" horizontalScrollPolicy="off"
borderStyle="solid" borderColor="0x888888" borderThickness="3">
<mx:HBox width="100%">
<mx:Label id="firstNameLabel" text="{data.label}"/>
<mx:Label id="lastNameLabel" text="{data.url}"/>
</mx:HBox>
<mx:Text id="descriptionLabel" text="{data.side}"/>
</mx:VBox>
</mx:Component>
</mx:itemRenderer>
</mx:List>
</mx:Application>
borderVisible - это ничто во flex, используйте атрибуты borderStyle = solid, borderThickness и borderColor для отображения границы