system.xml.xmlnodelist doesn't exist

I am trying to port a project from PC to Windows Phone 8 but when I try and
build I get the error below;

Error building Player: Exception: Error: method System.Xml.XmlNodeList System.Xml.XmlNode::SelectNodes(System.String) doesn’t exist in target
framework. It is referenced from Assembly-CSharp.dll at System.Void
Atlas::LoadAtlas(System.String).

I am new to programming / scripting so could you point me in the right
direction please?

XMLDocument isn’t supported by windows phone API. Use XDocument instead.

using (StringReader reader = new StringReader(xmlDataString))
{
    XDocument doc = XDocument.Load(reader);
    IEnumerable<XElement> nodeList = doc.Descendants("nodeName");
    foreach (XElement node in nodeList)
    {
      dosomething();
    }
}