Make sure you are using MongoDB version 1.3 or newer.
findandmodify command which is only available in recent versions.
Add the gem to your Gemfile:
bundle install, create the indexes (and don't forget to do this
on your production database):
script/rails runner 'Delayed::Backend::Mongoid::Job.create_indexes'
To generate script/delayed_job:
rails generate delayed_job
That's it. Use delayed_job as normal.