Paperclip is distributed as a gem, which is how it should be used in your app.
Include the gem in your Gemfile:
gem "paperclip", "~> 3.1"
If you're still using Rails 2.3.x, you should do this instead:
gem "paperclip", "~> 2.7"
Or, if you want to get the latest, you can get master from the main paperclip repository:
gem "paperclip", :git => "git://github.com/thoughtbot/paperclip.git"
master is where active development happens and IS NOT RECOMMENDED for production use. Please use a gem or one of the version branches.
For Non-Rails usage, you must include the
Paperclip::Glue module into your model:
class ModuleName < ActiveRecord::Base include Paperclip::Glue ... end
When Rails loads, it will automatically add this module, so you don't have to worry about it there.