I had a right old ding dong with it when trying to install it into a rails app I'm working on, what with the documentation being a little out of date (I think) plus there being a load of confusing page on the net. In the end what worked for me was:
# sudo gem install chronic packet
# cd rails_app
# script/plugin install http://svn.devjavu.com/backgroundrb/trunk
# rake backgroundrb:setup
# rake db:migrate
So far this has worked for me...