ActiveAdmin : Make model compliant with ActiveModel and StrongParameters Thanks to @daxter (twitter @seanlinsley) for his help : https://github.com/gregbell/active_admin/pull/2326

admin_user.rb

# To support StrongParameters (require in Rails 4.0), add this (change names of
# fields of course) to your /app/admin/<model>.rb
# This example is for AdminUser model

ActiveAdmin.register AdminUser do
  controller do
    def permitted_params
      params.permit admin_user: [:email, :password, :password_confirmation]
    end
  end
end

Gemfile

# Rails 4.0 just went out and right now to make ActiveAdmin works, you need 
# to add this in your Gemfile
gem 'devise',              github: 'plataformatec/devise'
gem 'responders',          github: 'plataformatec/responders'
gem 'inherited_resources', github: 'josevalim/inherited_resources'
gem 'ransack',             github: 'ernie/ransack', branch: 'rails-4'
gem 'activeadmin',         github: 'gregbell/active_admin', branch: 'rails4'
gem 'formtastic',          github: 'justinfrench/formtastic'