I wouldn't worry about what the changes and sources files do right now. 
You want to know how to open the image and look at the code. These 
videos will help with that. [1] You need to go to Squeakland [2] and get 
yourself an application. You can open the Scratch image from the Scratch 
website with that. Finally, there is a beginners list [3] you can ask 
questions at. This list isn't the best for this kind of question. Once 
you learn some basics of Smalltalk, you should be able to explore the 
source code of Scratch.

Chris


[1] http://smalltalk.org/videos/video_AbsoluteBeginner_DownloadOpen.html
[2] http://www.squeakland.org
[3] http://lists.squeakfoundation.org/mailman/listinfo/beginners