Caricamento di marcatori da file XML all'API di Google Map

non stai chiamando loadXMLFile()
non stai includendo la libreria jquery

non stai creando google.maps.Markers (sembra che tu abbia invece la sintassi di alcune API Nokia).

<!DOCTYPE html>
<html>
<head>
<title>Simple Map</title>
<meta name="viewport" content="initial-scale=1.0, user-scalable=no">
<meta charset="utf-8">
<style>

  html, body, #map-canvas
  {
    height: 100%;
    margin: 0px;
    padding: 0px
  }
</style>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script src="https://maps.googleapis.com/maps/api/js?v=3.exp&sensor=false&libraries=places"></script>
<script>

var map = null;
function initialize()
{

   var mapOptions =
       {
            zoom: 12,
            center:new google.maps.LatLng(53.3478, -6.2597)//center over dublin
       };

   map = new google.maps.Map(document.getElementById('map-canvas'), mapOptions);
   loadXMLFile();
 }

 function loadXMLFile(){
    var filename = 'v3_SO_20140124_markers.xml';
    $.ajax({
            type: "GET",
            url: filename ,
            dataType: "xml",
            success: parseXML,
            error : onXMLLoadFailed
    });

  function onXMLLoadFailed(){
    alert("An Error has occurred.");
  }

  function parseXML(xml){
    var bounds = new google.maps.LatLngBounds();
     $(xml).find("marker").each(function(){
            //Read the name, address, latitude and longitude for each Marker
            var nme = $(this).find('name').text();
            var address = $(this).find('address').text();
            var lat = $(this).find('lat').text();
            var lng = $(this).find('lng').text();
            var markerCoords = new google.maps.LatLng(parseFloat(lat), 
                                                      parseFloat(lng));
            bounds.extend(markerCoords);
            var marker = new google.maps.Marker({position: markerCoords, map:map});
        });
        map.fitBounds(bounds);
    }
}
google.maps.event.addDomListener(window, 'load', initialize);
</script>

esempio funzionante