[quazare]

public var externalXML:XML=new XML();	
public var cards:Array =new Array();

//----------------------------инициализация-----------------------	
	private function initApp():void { 
		mdm.Application.init(this);    
		
		var loader:URLLoader = new URLLoader();
		var request:URLRequest = new URLRequest("xmlFile.xml");
		loader.load(request);
		
		loader.addEventListener(Event.COMPLETE, onComplete)
		
		XMLtoArr();
	}

private function XMLtoArr():void{
	
	//for (var i:Number=0; i<;i++){
	cards[0]=externalXML.children().length();
	Alert.show(cards[0]);
	//}

}
	
private function onComplete(event:Event):void {
    var loader:URLLoader = URLLoader(event.target);
    externalXML = XML(loader.data);
}

вообще проблема в функции XMLtoArr()...