look this simple example :-
function ReadXml(retval,AD8)
{
var xmldoc=new ActiveXObject("Microsoft.XMLDOM");
xmldoc.loadXML(retval);
var node=xmldoc.getElementsByTagName('UrFieldName');
if (node[0]!=null) AD8.value=node[0].childNodes[0].data;
}
retval is your xml file data value.