sudo gem install ruby-mbox

then I run this

require "rubygems"
require "ruby-mbox"

mbox = Mbox.new("Inbox")

and I get this

$ ruby mbox_read.rb 
/usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require': no such file to load -- ruby-mbox (LoadError)
    from /usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
    from mbox_read.rb:2

gem server says this

ruby-mbox 0.0.2 [rdoc] [www]
A simple library to read mbox files. 
Executables are mbox-daemon, mbox-do. 

Thanks in advance