/NotificationMessage/Body/Values/*/text()

(найти Expression1 и Expression2 в виде XElement, обратиться к свойству Value), использование XPathDocument, итд:
var xd = new XmlDocument();
xd.Load(@"D:\Trash\1.xml");
foreach(XmlCDataSection n in xd.SelectNodes("/NotificationMessage/Body/Values/*/text()"))
  Console.WriteLine(n.Value);
Console.WriteLine("=============================");
var xd2 = XDocument.Load(xd.CreateNavigator().ReadSubtree());
foreach(var n in xd2.XPathSelectElements("/NotificationMessage/Body/Values/*"))
  Console.WriteLine(n.Value);
Console.WriteLine("=============================");
var xd3 = new XPathDocument(xd.CreateNavigator().ReadSubtree());
var iterator = xd3.CreateNavigator().Evaluate("/NotificationMessage/Body/Values/*/text()") as XPathNodeIterator;
foreach (XPathItem n in iterator)
  Console.WriteLine(n.Value);