Gnucleus is pretty decently commented, anywhere things get complicated there are definently comments to help people out. When ever I write code I think to myself, 'if someone reads this, will they understand it?' If you do have trouble with the source email me, I'll explain it all to you, whatever it is.
Go to
http://www.gnucleus.net , on the left hand side there are a set of links to the developer section. I suggest you (Moak) first take a look at the 'Structure' link, this will help you or anyone else understand the basic class structure of gnuclues.
If you open the workspace in MS dev, you will be able to use some of visual studio's advanced source browsing features to get a better feal of the code and how classes are organized.