I'm glad it works for you now.
Uncommenting the BOARD_TAG and and other variables in the root makefile and running a "make build" from the Terminal gave me that clue.
The BOARD_TAG variable is defined when you select the board on Info pane. Please refer to section 4.2. Change the Board on the User Manual.
(btw, is there any way to make XCode give you the verbose build output. It would give better clues, sometimes).
All the build, link and upload commands as well as the error messages are available on the Issue navigator. Please refer to section 5.4. Error Messages on the User Manual.
Xcode is a professional IDE and requires some learning. I strongly recommend Xcode 4 Unleashed
by Fritz F. Anderson I bought thanks to contributions I've received.