The XCode Cocoa Application Tutorial states that one has to create a controller class (ConverterController in this tutorial). This controller needs to be included into the Interface Builder (IB) by "Open->Read Class File".
If this is done, there will be an error in the MainMenu XIB/NIB-Window that says: "1 source file parsed, no classes found or changed".
I'm not sure what the problem is (maybe there was a change for Snow Leopards and XCode 3.2.1), but I found a way on how to import the Class file. You need to select "Tools->Library" and select the "Classes" tab in the Library window. You will find the ConverterController class in this list. Double-Click it and the class will appear in your MainMenu.xib