Make sure you are using MongoDB version 1.3 or newer. delayed_job_mongoid uses the findandmodify command which is only available in recent versions.

Add the gem to your Gemfile:

gem 'delayed_job_mongoid'

After running 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.