As with the other language bindings, the Ruby bindings are auto-generated and include a Makefile that uses extconf.rb. These bindings are only available for Linux and MacOSX. (We would welcome help in making them available on Windows also.)
To generate the Makefile and run it, use something like the following:
% ruby extconf.rb --with-openbabel-include=/usr/local/include/openbabel-2.0
% make
Like any Ruby module, the Open Babel bindings can be used from a Ruby script or interactively using irb as follows:
% irb
irb(main):001:0> require 'openbabel'