Maybe it's a glitch, it wasn't working for me either. Then I created a new Xcode project from scratch, and in that one was working fine.
I came back to the one that wasn't working and now it started accepting changes.
Make also sure that the column "Complier Flags" it's not hidden to the far right.
Edit:I don't know if it's your case but there's definitely a glitch. If you load the project settings in a narrow window (say in assistant mode) the flags column won't appear, and the popup text where you are supposed to enter the flags won't show. If I go to standard editor, then it behaves normally.