User1508394307 posted
You would meed to add namespace like this
//Instantiate an XmlNamespaceManager object.
System.Xml.XmlNamespaceManager xmlnsManager = new System.Xml.XmlNamespaceManager(xmldoc.NameTable);
//Add the namespaces used in books.xml to the XmlNamespaceManager.
xmlnsManager.AddNamespace("bb0", "urn:Books");
See here complete example http://support.microsoft.com/kb/318545/en