Why can't I "edit" (open in Interface Builder) a carbon .nib in Xcode 4.6.1?

Question

I can't believe I need to ask this...

I'm upgrading a Carbon app from Xcode 3.4.x to Xcode 4.6.1.

When I select a .nib file in the Project Navigator, it only wants to open/ display with "Preview". I can't get the Interface Builder Editor to display -- at all!

Obviously, there is some magic here. Do the .nib files need to be converted to .xib first? Or is there some component that needs to be installed that wasn't? (I think the command line tools may not be installed).

UPDATE: I've converted one of the .nib files to .xib, and dropped it back into the project. No difference: I still cannot bring up Interface Builder (at all).

Answer 1

Xcode 4 does not support editing Carbon nibs, though you can build an app using them. For editing the nibs, you'd better have a Mac that can run Snow Leopard, or Snow Leopard Server in a virtual machine, with Xcode 3.2.6 installed on it.

Answer 2

Continuing my answer from the reply on JWWalker's answer, on how to change the interface builder version: