UPDATE: Starting Nov 6 2010, Ymacs moved to GitHub: http://github.com/mishoo/ymacs/. The Mercurial repo will stay for a while but won't be updated any longer and will, at some point, disappear.
We're using Mercurial for source code management. You can browse the repository online. To work on it, you'd better clone the repository on your local machine:
hg clone http://code.ymacs.org/hgwebdir.cgi/ymacs/
git clone https://github.com/mishoo/ymacs.git
Then, if you point your browser to ymacs/test/index.html it should open a demo. Note that if you opened the demo through a “file://”-like URL, then the first menu (“Load its own code”) won't work. You need to put it behind a real HTTP server, such as Apache, in order for that menu to work, because it uses XMLHttpRequest which is forbidden for local URLs.
If something is fishy, please let me know. On the other hand, if you made some interesting changes that you would like to see in the official repository, please please let me know. :-)