You’ve implemented the Undo action, go to add the redo and… there is nothing there.
Add the following to any files .h, connect it up in the Nib / Storyboard and then you can delete it.
@interface NSResponder (Redo) - (IBAction)redo:(id)sender; @end
This should work for any action you want. For some reason it’s a bug in Xcode with no fix at the time of writing.