I found this about the "gem" command within IRB:

Kernel#gem
gem(gem_name, *version_requirements)
------------------------------------------------------------------------
     Adds a Ruby Gem to the $LOAD_PATH. Before a Gem is loaded, its
     required Gems are loaded. If the version information is omitted,
     the highest version Gem of the supplied name is loaded. If a Gem
     is not found that meets the version requirement and/or a required
     Gem is not found, a Gem::LoadError is raised. More information on
     version requirements can be found in the Gem::Version
     documentation.

     The gem directive should be executed before any require statements
     (otherwise rubygems might select a conflicting library version).

     You can define the environment variable GEM_SKIP as a way to not
     load specified gems. you might do this to test out changes that
     haven't been intsalled yet. Example:

       GEM_SKIP=libA:libB ruby-I../libA -I../libB ./mycode.rb

     gem:                 [String or Gem::Dependency] The gem name or
                          dependency instance.

     version_requirement: [default=">= 0.0.0"] The version requirement.

     return:              [Boolean] true if the Gem is loaded,
                          otherwise false.

     raises:              [Gem::LoadError] if Gem cannot be found, is
                          listed in GEM_SKIP, or version requirement
                          not met.

--

I'd guess that you haven't installed the gem correctly, so it isn't in 
the right location for "require" to work.

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