Assuming you already have brew installed, then:
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sudo brew install jruby. More Info: https://github.com/rubygame/rubygame/wiki/JRuby-Install-Guide
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For Eclipse we will need: Install Ruby (DLTK) 5.0 Kepler. More Info: http://marketplace.eclipse.org/content/ruby-dltk#.VAhLwGS1aaJ
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Test if jRuby it's working properly from the command line:
jruby -S gem list