Like many Apple developers, I was a developer on Windows platform before. It is very easy to add tool tip to a control (e.g. a button) in Visual Studio. But I don't know how to do it in Xcode. When I am working on iPhone apps, it is not a problem because iPhone apps need not tool tip. But it is a problem when I am working on Mac apps.
Actually it is easy to add tool tip in Xcode 4. Looks like Apple put it in a wrong place. You need to add tool tip in Identity Inspector.