MacRuby development happens in the trunk SVN branch. However, be careful, because trunk may not work (or even compile) all of the time.
$ svn co http://svn.macosforge.org/repository/ruby/MacRuby/trunk MacRuby-trunk
If you want to grab the sources of a specific release, look into the tags directory. For example, to grab the sources of the 0.5 release:
$ svn co http://svn.macosforge.org/repository/ruby/MacRuby/tags/0.5 MacRuby-0.5
For building instructions, please refer to README.rdoc file, but the basic build instructions are:
$ rake $ sudo rake install
You can also browse the repository online.
Git aficionados will be glad to learn that there is an official mirror of the SVN trunk repository available:
$ git clone git://git.macruby.org/macruby/MacRuby.git
Check out the MacRuby Roadmap