HotCocoa::Graphics is an addition to the MacRuby project, which is being developed by Apple and other contributors.  To use it, you will need to first install the MacRuby package from MacRuby.org, and then install the HotCocoa gem by typing the following in your terminal:

sudo macgem install hotcocoa

Or, install the standalone graphics library:

github.com/drtoast/macruby_graphics

HotCocoa::Graphics was written by James Reynolds, with contributions from Rich Kilmer and others.