Thanks, I see now that I've misunderstood how to subscribe to a service(read the manual would a colleague had said to me).

I modified the client code and added a subscription to "HelloWorldClient":

@EntryPoint

public class HelloWorldClient extends VerticalPanel {

  private int n = 0;

  @Inject

  private MessageBus bus;

  @PostConstruct

  public void init() {

    Button button = new Button("hello!");

    bus.subscribe("HelloWorldClient", new MessageCallback() {

                    @Override

                    public void callback(Message message) {

                              n++;

            Window.alert("From Server(messagenr:" + n + ") - " + message.get(String.class, MessageParts.Value));

                    }

    });

    button.addClickHandler(new ClickHandler() {

      public void onClick(ClickEvent event) {

        MessageBuilder.createMessage()

            .toSubject("HelloWorldService")

            .withValue("Hello, There!")

            .done()

            .sendNowWith(bus);

      }

    });

    add(button);

    RootPanel.get().add(this);

  }

}

and the server code to:

@Service

public class HelloWorldService implements MessageCallback {

  @Inject

  private MessageBus bus;

          @Override

          public void callback(Message message) {

        // Send a message to the 'HelloWorldClient'.

        MessageBuilder.createMessage()

            .toSubject("HelloWorldClient")             

            .signalling()                              

            .with("text", "Hi There")                  

            .noErrorHandling()                         

            .sendNowWith(bus);                  

          }

}

Now I get a response in the client but no messsage, how come?

Thanks a lot, working very nice!!!!