xmpp send, wait for response

Question

I'm programming in C# with XMPP chat protocol and want to wait in the GUI for the response of the counter part. It takes less then 1 seconds, till the counter part response (it's a script), so it's not a problem, if the GUI hangs for that time. I tried with ManualResetEvent, but I just get a timeout exception. I made following code (in extracts):

 using agsXMPP;
 using agsXMPP.protocol.client;
 using agsXMPP.Collections;
 using agsXMPP.protocol.iq.roster;
 using System.Threading;

 public partial class Form1 : Form
 {

   static string output_msg;
   private ManualResetEvent manualResetEvent;

   public Form1()
   {
     manualResetEvent = new ManualResetEvent(false);

     Jid jidSender = new Jid(username + "@" + server);
     xmpp = new XmppClientConnection(jidSender.Server);

     xmpp.UseStartTLS = true;
     xmpp.Open(jidSender.User, password, resource);
     xmpp.OnLogin += new ObjectHandler(OnLogin);

     // create message receive callback
     xmpp.MessageGrabber.Add(new Jid(JID_Receiver), new BareJidComparer(), new MessageCB(MessageCallBack), null);

     // send a message
     xmpp.Send(new agsXMPP.protocol.client.Message(new Jid(sendTo), MessageType.chat, message));

     // wait with ManualResetEvent, till response is available in output_msg
     if (!manualResetEvent.WaitOne(3000))
     {
       throw new TimeoutException();
     }
     manualResetEvent.Reset();

     // show response in a dataGridView . . .
     output.AppendText(output_msg);  // output is a TextBox, just for debugging
   }

   private void MessageCallBack(object sender, agsXMPP.protocol.client.Message msg, object data)
   {
      output_msg = msg.Body;

      manualResetEvent.Set();
   }
 }

How can I wait for response? Is there a alternative to ManualResetEvent?