The SKETCHBOOK_DIR should be pointed at your ~/Documents/Arduino path to take into account your personal libraries.
Once again, Xcode is a professional IDE and, as so, requires some learning.
I strongly recommend the book Xcode 4 Unleashed by Fritz F. Anderson I've purchased thanks to contributions.
Please keep in mind the embedXcode User Guide is not an introduction to Xcode. It implies some fluency with Xcode.
Enjoy 