Indeed, MusicKit is an audio tool, not a symbolic tool like you need.
I don't know of any open-source Objective-C music notation programs. However, the recently-released Zong! Viewer is an open-source (GPL) Java program. You can't run Java on the iPhone, of course, but it might give you some ideas for structuring your software. MuseScore is an open source music notation editor that might provide more ideas.
So far the only iPhone music notation application I know is pocketscore. It has the great idea of exporting MusicXML files via e-mail so you can work on your music later with a more full-featured program than you can write on an iPhone.
There might be other open source programs you can check out on the MusicXML software list. Good luck!
