Unknown class in Interface Builder file

This error can show up when using Storyboard or Interface Builder.  The problem is that no actual code references so the linker optimizes the class out of existence.  One solution to select the .m of the class that is giving you trouble.  Then show the file inspector (View->Utilities->Show file inspector) and check your target under the Target Membership section.

More info here:
http://stackoverflow.com/questions/1725881/unknown-class-myclass-in-interface-builder-file-error-at-runtime