If I'm not mistaken, your case is when you add them to an extension, you won't be able to access private vars/methods.
You have (what I suggest) two possible solutions:
1- If you want to keep them in a separated extension (but in the same .swift file) for the purpose of organizing your code, you need to change private to fileprivate.
For more information about access modifiers in Swift 3, you might want to check this answer.
2- Keep them in the ViewController and use // MARK: - IBActions, it marks sections in the symbol navigator.
For example:

As you can see, there are // MARK: - Core Data stack, // MARK: - Core Data Saving support and //MARK: - Machines Core Data to separate the code and make it more readable and organized.
Hope it helped.