That's because NSPopUpButton
inherits from NSButton
which is declared in the AppKit framework for OS X, and you can not use an OS X class in an iOS project.
(Basing this off the use of the iOS tag in the question)
If this is a Mac project, then you should check to make sure that your class is importing the AppKit framework, or Cocoa/Cocoa.h
which also imports AppKit.