really, your going to have to do some google and research. its not difficult code to read, if you know how to read arduino code.
http://www.youtube.com/watch?v=pUYKWm_4KZQmaybe start with a video like this, or just go to the arduino home page, and look up info as you read the multiwii code.
the code is pretty well documented throughout the sketch, and thoughout this site. the advanced search engine works really well too.