I am currently making an app that allows third-party developers to create widgets for it using the API. Right now, I ship a library with the application containing a class that developers must create a subclass of. They must package their program in an NSBundle for my app to load.
Have you ever created an Objective-C APIs? If so, do you have tips to tell me?
If your goal is to provide documentation for your API I would suggest you to use doxygen. Here you'll find an interesting tutorial on how to document with doxygen.
I would also recommend you reading API Design by Matt Gemmell. It's a good reference on what a developer could expect from a well designed API.