Namespace Manager or XsltContext needed. This query has a prefix, variable, or user-defined function.

if you got this error dealing with XML

{"Namespace Manager or XsltContext needed. This query has a prefix, variable, or
user-defined function."}

You need to make sure that you include the XmlNamespaceManager

//Setting up NSManager
XmlNamespaceManager man = new XmlNamespaceManager(rssDoc.NameTable);
man.AddNamespace("msncp", "urn:schemas-microsoft-com/contentpublishing/content");

take a look at the example here
http://carso-owen.blogspot.com/2009/02/how-to-get-data-from-msn-navigation.html