rake problemmm?

Hello RoR Developers,

I have problem with rake, I got gettext.rake("lib/tasks") something like

desc 'Create mo files'
  task :createmo do
    require 'gettext/utils'
    require 'gettext/rails'

    GetText.create_mofiles(true, 'po', 'locale')

when I try to use it,gives me error like,
rake createmo

(in C:/..../rails)
rake aborted!
Don't know how to build task 'createmo'

(See full trace by running task with --trace)

what can I do now? what is the problem here..