How to fix "Cannot find interface declaration for 'NSManagedObject', superclass of"?

Try

#import <CoreData/CoreData.h>

Are you missing an include statement?

Are you subclassing NSManagedObject? If so, go here in Apple's documentation and scroll to Xcode Generated Subclasses. They give you instructions to have Xcode generate class files from your data model.

A suggestion: Core Data is rather difficult, and you should be comfortable with iPhone development before you start working with it.