Add the gems to your Gemfile:
gem 'delayed_job'
gem 'delayed_job_mongo_mapper', :git => "git://github.com/thisduck/delayed_job_mongo_mapper.git"
After running bundle install, create the indexes (and don't forget to do this on your production database):
rails runner 'Delayed::Backend::MongoMapper::Job.create_indexes'
That's it. Use delayed_job as normal.
This is forked and modified from delayed_job_mongoid.