Did you mean Rails 3.2.3?
Then you have to use the next_stable branch:
In your Gemfile:
gem 'alchemy_cms', :git => 'git://github.com/magiclabs/alchemy_cms', :branch => 'next_stable'
Greetings
Thomas
But this is not a problem, you can create a vanilla rails app and add the next_stable branch into you Gemfile like I mentioned earlier.
Then you just have to follow the steps for adding alchemy into an existing rails app:
http://guides.alchemy-cms.com/getting_started.html#install-into-an-existing-rails-application
Hope this helps.
Regards
Thomas