Can't Parse XML data with C# [Nightmare]

i have been trying to extract one value from a XML file, and then store it on the same file but in another node, i tried all the examples i've found on the net, read XPath Syntax documentation like hell and still can't get it to work.

I must take the ID Value (which may vary) and copy it into .

But i can't even do that if i can't manage to parse the lines, most of times node/variables/"", or i get an "object reference not stablished as object instance error".

Here's the code:

    // Create a new XML document.
        XmlDocument xmlDoc = new XmlDocument();
        // Load an XML file into the XmlDocument object.
        xmlDoc.PreserveWhitespace = true;
        xmlDoc.Load(pfile);

        //TEST !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! PROBLEMA

        XmlNodeList Documentos = xmlDoc.GetElementsByTagName("//Documento");
        XmlNodeList DatosDocumento = ((XmlElement)Documentos[0]).GetElementsByTagName("ID");

        foreach (XmlElement nodo in DatosDocumento)
        {

            int i = 0;

            XmlNodeList ID = nodo.GetElementsByTagName("ID");
            Console.WriteLine("Elemento nombre ... {0}}", ID[i].InnerText);

        }

        //
        XmlNodeList nodes = xmlDoc.SelectNodes("EnvioDTE");
        XmlNode nodesimple = xmlDoc.SelectSingleNode("EnvioDTE");
        Console.WriteLine("Lista Nodos: " + nodes.Count);
        Console.WriteLine("Nodo Simple: " + nodesimple.InnerText);
        foreach (XmlNode node in nodes)
        {
            string id = node.Attributes["ID"].InnerText;
            Console.WriteLine(id);

        }

I am almost certain the problem is on the XPath Syntax, but i can't get it to work.

sadly i can't use XDocument as im using .NET 3.5 for this task, i would really appreciate some help on this, by behand apologize my bad english

As the XML file is too big, il put it here on this URL http://ift.tt/1qX36yB

Thanks in advance