error with import <cocoa/cocoa.h>

I added class files from another project to my new iPhone Window for the first time. The class is a simple class that defines a polygon, and subclasses NSObject. As part of the standard template for an Objective C class, this class definition imports Cocoa.h

#import <Cocoa/Cocoa.h>

However, just by adding this class, I'm getting an error that

Cocoa/Cocoa.h:No such file or directory.

I don't understand this because the exact same line occurs in another class definition (the controller) within the same project.

From stackoverflow

• Subclasses of NSObject (at least on the iPhone) do not import the Cocoa.h header. Instead, they import Foundation.h:

  #import <Foundation/Foundation.h>
  

• On the iPhone you generally use UIKit instead of Cocoa, which is for Mac OS X.

  #import <UIKit/UIKit.h>
  

  You might import just the Foundation framework in a model class that doesn't reference any user interface stuff.

  #import <Foundation/Foundation.h>