How to: use carrierwave with rails 2.3.x - carrierwaveuploader/carrierwave GitHub Wiki
CarrierWave is only compatible with Rails 3 and later as of version 0.5. If you want to use Rails 2, it is recommended that you use the latest gem in the 0.4.X series. You may also consider trying this plugin which allows CarrierWave to be used with a Rails 2.3 app.