Here's the only AS3 code I'm using on the timeline:

var myXMLLoader:URLLoader = new URLLoader();
myXMLLoader.load(new URLRequest("twitter.php"));
myXMLLoader.addEventListener(Event.COMPLETE, processXML);
function processXML(e:Event):void {
var myXML:XML=new XML(e.target.data);
tweet_1.text=myXML.status[0].text;
}

followMe_btn.addEventListener(MouseEvent.CLICK, onFollow);
function onFollow(e:MouseEvent):void{
navigateToURL(new URLRequest("http://twitter.com/big_pearl"));
}

Obviously, the onFollow function works. It's the processXML function that's the issue.