Given that you've resolved the Load
/LoadXml
confusion, I expect the issue is namespaces; do you have example xml? Handling xml with namespaces gets... "fun" ;-p
For example:
XmlDocument doc = new XmlDocument();
doc.LoadXml(@"<test xmlns='myFunkyUri' value='abc'/>");
// wrong; no namespace consideration
XmlElement root = (XmlElement)doc.SelectSingleNode("test");
Console.WriteLine(root == null ? "(no root)" : root.GetAttribute("value"));
// right
XmlNamespaceManager nsmgr = new XmlNamespaceManager(doc.NameTable);
nsmgr.AddNamespace("x", "myFunkyUri"); // x is my alias for myFunkyUri
root = (XmlElement)doc.SelectSingleNode("x:test", nsmgr);
Console.WriteLine(root == null ? "(no root)" : root.GetAttribute("value"));
Note that even if your xml declares xml aliases, you may still need to re-declare them for your namespace-manager.