Undefined symbols for architecture x86_64 on Xcode 6.1

Apparently, your class "Format" is involved in the problem. Check your declaration of this class, especially if you did it inside another class you probably forgot the @implementation or something similar.

Check if that file is included in Build Phases -> Compiled Sources

Make sure the WOExerciseListViewController is a Target Member; that worked for me!

Yes, I think the library you are using is not compatible with 64 bit version but you can solve the problem -

Just navigate to Build Settings>Architectures & replace $(ARCHS_STANDARD) to $(ARCHS_STANDARD_32_BIT)

So that your xcode build your project with 32 bit supported version.