Mac OS X actually comes with Ruby and RubyGems built straight into the operating system already. The only additional software you need to install Vagrant is Xcode, which you can get for free in the Mac App Store.
Also, you will want to update your Rubygems installation, since Vagrant requires a slightly newer version than what is distributed with Mac OS X:
$ sudo gem update --system
Now just head back to the getting started overview page and continue with the guide!