OK,
I think this is what I did. I'm new at Xcode too...
Now I tried again, and I gota new list of 18 errors (I think in my first trial I accepted to change some settings I didn't understood 
So, I started from scratch (downloading openframeworks again), and I just changed the Base SDK in openFrameworksLib.xcodeproj. Got these 18 errors, and "Build openframeworks failed" message.
Is it possible that I have to change some settings first? Where can I find information? I'm lost!
Thanks,