The easiest way is to use cmake and let it create the Xcode project, this has the advantage that the dictionaries are created automatically. The CMakeLists.txt file will define the libraries and executables to be added to the Xcode project. An example from the ROOT Event example (found in root/test) can be downloaded here. Just un-tar the attached file and run
cd event/build cmake -G Xcode .. xcodebuild
This will create an Xcode project event.xcodeproj which can be opened and used by Xcode.