Multiplayer not working

Hi I am trying to set up a multiplayer game. So i followed a tutorial (http://cgcookie.com/unity/2011/12/20/introduction-to-networking-in-unity/) and made a first test. Now i can connect with myself and it works just fine, but when my friend tries to connect, things get weird. Usually one player starts a server, and the other clicks the refresh hosts button. Then a button appears for each available host/server. When i test it on my own, i can connect by clicking on this new button. When my frined tries it however, nothing happens. The same situation when he hosts and i try to connect.

This is the full Network Script:

 #pragma strict
 
 var gameName : String = "TestMultiplayerGame123321";
 private var refreshing : boolean;
 private var hostData : HostData[];
 
 var Button01 : GameObject;
 var Button02 : GameObject;
 
 var player : GameObject;
 var spawn : GameObject;
 
 //Start Server
 function StartServer()
 {
     Network.InitializeServer(2, 25000, !Network.HavePublicAddress);
     MasterServer.RegisterHost(gameName, "Test", "This is a test");
 }
 
 //Messages
 function OnServerInitialized()
 {
     Debug.Log("Server Initialized");
     spawnPlayer();
 }
 
 function OnConnectedToServer()
 {
     spawnPlayer();
 }
 
 //Spawning a Player
 function spawnPlayer()
 {
     Network.Instantiate(player, spawn.transform.position, Quaternion.identity, 0);
 }
 
 function OnMasterServerEvent(mse : MasterServerEvent)
 {
     if (mse == MasterServerEvent.RegistrationSucceeded)
     {
         Debug.Log("Registration Succeeded");
     }
 }
 
 //Refreshing Hosts etc. 
 function RefreshHostList()
 {
     MasterServer.RequestHostList(gameName);
     refreshing = true;
 }
 
 function Update()
 {
     if (refreshing)
     {
         if(MasterServer.PollHostList().Length > 0)
         {
             refreshing = false;
             Debug.Log(MasterServer.PollHostList().Length);
             hostData = MasterServer.PollHostList();
             //ListHosts();
         }
     }
 }
 
 function OnGUI()
 {
     if (!Network.isClient && !Network.isServer)
     {
         if (hostData != null)
         {
             for (var i : int = 0; i < hostData.length; i++)
             {
                 if(GUI.Button(Rect(Screen.width * 0.5, Screen.height * 0.1 + i/2, 50, 50), hostData[i].gameName))
                 {
                     Network.Connect(hostData[i]);
                 }
             }
         }
     }
     else
     {
         Button01.active = false;
         Button02.active = false;
     }
 }

Can anyone help me?

Thanks in advance :-)

Btw, don´t let the usage of both the new GUI system and the old one confuse you. I was experiencing some problems so i alternated between both for test purposes