[etc]

var listXML = new XML();
listXML.ignoreWhite = true;
listXML.onLoad = function(success) {
	if (!this.loaded||this.status) {
		throw new Error('An error occured while loading XML');
		return;
	}
	var nodes = this.firstChild.childNodes;
	var nodesLength = nodes.length;
	for (var i = 0;i<nodesLength;i++) {
		var node = nodes[i];
		list_cmb.addItem(node.attributes['label'],node.childNodes);
	}
	list_cmb.dispatchEvent({type:'change'});
}
var comboboxListener = {};
comboboxListener.change = function(event) {
	var nodes = event.target.selectedItem.data;
	var nodesLength = nodes.length;
	this.items_lst.removeAll();
	for (var i = 0;i<nodesLength;i++) {
		var node = nodes[i];
		this.items_lst.addItem(node.attributes['label'],node);
	}
}
comboboxListener.items_lst = items_lst;
var listListener = {};
listListener.change = function(event) {
	trace('URL: '+event.target.selectedItem.data.attributes['url']);
}
items_lst.addEventListener('change',listListener);
list_cmb.addEventListener('change',comboboxListener);
listXML.load('asd.xml');

asd.xml:

<?xml version="1.0" encoding="utf-8"?>
<strings>
	<list label="1">
		<item label="1_1" url="1_1.txt"/>
		<item label="1_2" url="1_2.txt"/>
		<item label="1_3" url="1_3.txt"/>
	</list>
	<list label="2">
		<item label="2_1" url="2_1.txt"/>
		<item label="2_2" url="2_2.txt"/>
		<item label="2_3" url="2_3.txt"/>
	</list>
	<list label="3">
		<item label="3_1" url="3_1.txt"/>
		<item label="3_2" url="3_2.txt"/>
		<item label="3_3" url="3_3.txt"/>
	</list>
</strings>

items_lst — List на сцене
list_cmb — ComboBox на сцене

Все-таки мне пришлось сделать за вас вашу работу.