In "irb", type:
require 'selenium-webdriver'
to load gem 'selenium-webdriver';
if your Ruby version is lower than 1.9.x, you need to require 'rubygems'
before this step to load "rubygem" so that it can load other gems, for Ruby 1.9+, "rubygem" is automatically loaded by Ruby. If you require 'rubygems'
on Ruby 1.9+, you will see following screen, just don't worry about it