X509Certificate2 xCert = null;

          X509Store store = new X509Store(StoreName.My, StoreLocation.CurrentUser);
          store.Open(OpenFlags.ReadOnly | OpenFlags.OpenExistingOnly);

          X509Certificate2Collection collection = (X509Certificate2Collection)store.Certificates;
          X509Certificate2Collection fcollection = (X509Certificate2Collection)collection.Find(X509FindType.FindByTimeValid, DateTime.Now, false);
          X509Certificate2Collection scollection = X509Certificate2UI.SelectFromCollection(fcollection, "Test Certificate Select", "Select a certificate from the following list to get information on that certificate", X509SelectionFlag.MultiSelection);

          Console.WriteLine("Number of certificates: {0}{1}", scollection.Count, Environment.NewLine);

          xCert = scollection[0];

          // Create a SignedXml object.
          SignedXml signedXml = new SignedXml(Doc);   //Doc экземпляр класса XmlDocument

          // Add the key to the SignedXml document.
          signedXml.SigningKey = xCert.PrivateKey;                  //<-- здесь вываливается NotSupportedException

          // Create a reference to be signed.
          Reference reference = new Reference();

          //...