As Ranjeet pointed out, that post is almost 4 years old. The latest FLTK may be quite different. There aren't a lot of Xcode specific details there, so that shouldn't be much of an issue.
I think you should focus on building the FLTK projects as you would build them on Linux - using only the command line. You don't really want to use Xcode at all. If Xcode is correctly installed, then everything should work the same way as it does on Linux. There may be some system paths that are different on MacOS X, but those can be easily tracked down an fixed.
Now, if you gain some additional experience with Xcode, you could certainly figure out how to integrated FLTK projects into Xcode in a more user-friendly manner. I doubt it would be worth it though. If you want to make a professional-looking MacOS X program, you need to use Xcode and Interface Builder. You can re-use common code used by other operating systems, but the GUI and system-specific tasks need to be done by the vendor's (Apple's) tools for best results. Since GUI is all the FLTK does, I wouldn't recommend using it on MacOS X.