After updating to Xcode 5.1, I can no longer build my project for the 64-bit simulator, receiving this error:
No architectures to compile for (ONLY_ACTIVE_ARCH=YES, active arch=x86_64, VALID_ARCHS=i386).
I had the same error message after upgrading to XCode 5.1. Are you using CocoaPods? If so, this should fix the problem: