Here is a blog on how to do XPATH with namespaces (which BizTalk normally adds):
http://nealwalters.blogspot.com/2005/01/common-xpath-examples-questions-issues.html
I suggest using a tool like StylusStudio or XML-Spy to test your XPath, then copy/paste a working one into BizTalk. This can save a lot of time!
Here is a brief blog on how to use XPATH in a Biztalk Expression shape. As it says, don’t forget the keyword “string()” around your xpath. Also see BizTalk documentation for how this works.
http://geekswithblogs.net/andym/archive/2006/12/06/100475.aspx
As far as I know, the XPath command in BizTalk cannot add nodes. You have to call a C# helper class.
Here are two examples of how to add a node to an XML document using a C# helper method.
http://biztalk-training.com/administration/articles.php?article_id=14
Neal Walters
http://BizTalk-Training.com – Learn BIzTalk with Videos