Interface mapping problem/question

I am trying a very very simple webservice implementation, and still I do not succeed :-((

I have done all definition for the data types, message types, message interfaces and message mappings. The last step is to create the interface mapping.

I have a simple webservice which can increment an integer, (whow!!), and now I want to use it from a client app, via XI, just as the calculator example in Siva Maranani's Blog (Invoke Webservices using SAPXI).

Creating the Interface Mapping I do:

Source Interface: the Counter_Output_MI

Target Interface: the Counter_Input_MI

Then I press the Read Interfaces button.

In the Request Tab the source message is populated with the Counter_Output_MT, while I expected the Input Message Type.

I checked and rechecked everything a hundred times, but I can not find what I did wrong....