uninitialized constant ActionController::Caching::Fragments

Hey, i'm trying to use "s3cache" plugin with my app, but after adding it
to my vendor/plugins directory and following the install steps i'm
getting this error after running script/server:

/Users/richardschneeman/Documents/AptanaStudio/slangasaurus/vendor/rails/activesupport/lib/active_support/dependencies.rb:275:in
`load_missing_constant': uninitialized constant
ActionController::Caching::Fragments::UnthreadedFileStore (NameError)
  from
/Users/richardschneeman/Documents/AptanaStudio/slangasaurus/vendor/rails/activesupport/lib/active_support/dependencies.rb:467:in
`const_missing'
  from
/Users/richardschneeman/Documents/AptanaStudio/slangasaurus/vendor/plugins/s3cache/lib/s3_cache.rb:3
  from
/opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in
`gem_original_require'
  from
/opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in
`require'
  from
/Users/richardschneeman/Documents/AptanaStudio/slangasaurus/vendor/rails/activesupport/lib/active_support/dependencies.rb:509:in
`require'
  from
/Users/richardschneeman/Documents/AptanaStudio/slangasaurus/vendor/rails/activesupport/lib/active_support/dependencies.rb:354:in
`new_constants_in'
  from
/Users/richardschneeman/Documents/AptanaStudio/slangasaurus/vendor/rails/activesupport/lib/active_support/dependencies.rb:509:in
`require'
  from
/Users/richardschneeman/Documents/AptanaStudio/slangasaurus/vendor/plugins/s3cache/init.rb:1:in
`evaluate_init_rb'
   ... 37 levels...
  from
/Users/richardschneeman/Documents/AptanaStudio/slangasaurus/vendor/rails/activesupport/lib/active_support/dependencies.rb:509:in
`require'
  from
/Users/richardschneeman/Documents/AptanaStudio/slangasaurus/vendor/rails/railties/lib/commands/server.rb:39
  from script/server:3:in `require'
  from script/server:3

I've extensively googled this error but haven't been able to come up
with any good answers for what is causing it. Rails 2.1.0, Ruby 1.8.6:
this is the first two lines in s3_cache.rb:

require 's3'
class S3Cache <
ActionController::Caching::Fragments::UnthreadedFileStore

Any clues or suggestions??

I've extensively googled this error but haven't been able to come up
with any good answers for what is causing it. Rails 2.1.0, Ruby 1.8.6:
this is the first two lines in s3_cache.rb:

require 's3'
class S3Cache <
ActionController::Caching::Fragments::UnthreadedFileStore

Your plugin was written against 2.0.2. That class doesn't exist in
rails 2.1

FGred