Follow the Instructions from the Ruby Installer Developer Kit Wiki:
cd C:\Ruby193\DevKit
ruby dk.rb init
ruby dk.rb review
ruby dk.rb install
To return to the problem at hand, you should be able to install JSON (or otherwise test that your DevKit successfully installed) by running the following commands which will perform an install of the JSON gem and then use it:
gem install json --platform=ruby
ruby -rubygems -e "require 'json'; puts JSON.load('[42]').inspect"