Just follow the guide to get up and running:
git clone git://github.com/Emerson/Multiple-File-Uploads-with-Paperclip-and-Rails-3.git paperclip-rails3
cd paperclip-rails3
bundle install
rake db:migrate
rails server
Be sure to checkout the screencast at http://www.emersonlackey.com/article/paperclip-with-rails-3