If your XML contains namespaces, then you can do the following in order to obtain the value of an attribute:
var xmlDoc = new XmlDocument();
// content is your XML as string
xmlDoc.LoadXml(content);
XmlNamespaceManager nsmgr = new XmlNamespaceManager(new NameTable());
// make sure the namespace identifier, URN in this case, matches what you have in your XML
nsmgr.AddNamespace("ns", "urn:oasis:names:tc:SAML:2.0:protocol");
// get the value of Destination attribute from within the Response node with a prefix who's identifier is "urn:oasis:names:tc:SAML:2.0:protocol" using XPath
var str = xmlDoc.SelectSingleNode("/ns:Response/@Destination", nsmgr);
if (str != null)
{
Console.WriteLine(str.Value);
}