MacRuby development happens in the master branch of our GitHub repository. However, be careful, because it may not work (or even compile) all of the time.
$ git clone git://github.com/MacRuby/MacRuby.git macruby
If you want to grab the sources of a specific release, look for the specific branch. For example, to grab the sources of the latest release:
$ git clone -b 0.10 git://github.com/MacRuby/MacRuby.git macruby-0.10
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.