ARC is only available to 64-bit applications in OS X. Therefore, you must set your application's architecture to "64-bit Intel" in the build settings. If you want to support 32-bit OS X systems then you cannot use ARC.
"Fragile ABI" refers to the old "Legacy" Objective-C runtime, which is what 32-bit OS X applications use. iOS and 64-bit OS X apps use the "Modern" Objective-C runtime.
