Declare proper prefix to namespace URI mapping, then you can use XPath query using that prefix plus specifying element index to select the second <workshop1>
element, for example :
XmlDocument doc = new XmlDocument();
doc.Load("C:\\test.xml");
XmlNamespaceManager oManager = new XmlNamespaceManager(doc.NameTable);
oManager.AddNamespace("ns", "http://schemas.microsoft.com/sqlserver/reporting/2005/01/reportdefinition");
var google = doc.SelectSingleNode("/ns:Report/ns:buttons/ns:workshop1[2]", oManager).InnerText;