Use strString( string variable) not xmldoc .
Use below code in expression shape.
strString= xpath(TypedPollingProcRsp, string(string(/*[local-name()='TypedPolling' and namespace-uri()='http://schemas.microsoft.com/Sql/2008/05/TypedPolling/Test']/*[local-name()='TypedPollingResultSet0' and namespace-uri()='http://schemas.microsoft.com/Sql/2008/05/TypedPolling/Test']/*[local-name()='TypedPollingResultSet0' and namespace-uri()='http://schemas.microsoft.com/Sql/2008/05/TypedPolling/Test'][2]/*[local-name()='Data' and namespace-uri()='http://schemas.microsoft.com/Sql/2008/05/TypedPolling/Test'])))
Thanks
Abhishek