The way I work with the demos is:
1) Create new Xcode project
2) Add Libraries to link with (click project, go to targets, Build Phases tab, "plus" Link With Libraries)
3) Copy .cpp and .h files from the demo I am interested in (say VehicleDemo for example)
4) Most demos require the .cpp and .h files from Demos/OpenGL... So I copy all of those into my project.
5) I get errors on the "btBulletDynamicsCommon.h" because of how frameworks are structured, vs an includes directory...
So I modify the #include line in the files that need it as follows:
I hope that helps some. Cmake did not do the trick for me, but it turns out to be pretty easy to copy the files into a fresh project to get things working as described above.