The FSEvents notification mechanism is a runloop source, sitting around waiting for stuff to happen. A C callback function is called when a message from fseventsd shows up. The FSEventsStreamRef type is what you use to reference a particular event stream.
Before you can start receiving events, you need to create the event stream with one of these two functions:
FSEventStreamRef FSEventStreamCreate (CFAllocatorRef allocator, FSEventStreamCallback callback, FSEventStreamContext *context, CFArrayRef pathsToWatch, FSEventStreamEventId sinceWhen, CFTimeInterval latency, FSEventStreamCreateFlags flags);
FSEventStreamRef FSEventStreamCreateRelativeToDevice (CFAllocatorRef allocator, FSEventStreamCallback callback, ...