Cannot start server with "rack-recaptcha" gem - ruby-on-rails

Question

I have followed steps, described in rack-recaptcha readme.
I have done all the steps, instead of "You have to require 'rack-recaptcha' in your gemfile."
Server is unable to start, because of a following error:
Booting Mongrel
=> Rails 2.3.5 application starting on http://127.0.0.1:3001
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:440:in `load_missing_constant'
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:80:in `const_missing'
/{path_to_my_project}/config/environment.rb:29
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/initializer.rb:111:in `run'
/{path_to_my_project}/config/environment.rb:9
/Library/Ruby/Site/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
/Library/Ruby/Site/1.8/rubygems/custom_require.rb:31:in `require'
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:156:in `require'
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:521:in `new_constants_in'
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:156:in `require'
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/commands/server.rb:84
/Library/Ruby/Site/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
/Library/Ruby/Site/1.8/rubygems/custom_require.rb:31:in `require' ./script/server:3
/Library/Ruby/Gems/1.8/gems/ruby-debug-ide-0.4.9/lib/ruby-debug-ide.rb:109:in `debug_load'
/Library/Ruby/Gems/1.8/gems/ruby-debug-ide-0.4.9/lib/ruby-debug-ide.rb:109:in `debug_program'
/Library/Ruby/Gems/1.8/gems/ruby-debug-ide-0.4.9/bin/rdebug-ide:87
/usr/bin/rdebug-ide:19:in `load'
/usr/bin/rdebug-ide:19
-e:2:in `load'
-e:2
Uncaught exception: uninitialized constant Rack::Recaptcha
I think, the step with "gemfile", that i'm skipped, is the reason of this error.
But, i don't know, where should i look for the mentioned "gemfile" ?
Help, please
Update:
environment.rb is following
# Be sure to restart your server when you modify this file
# Specifies gem version of Rails to use when vendor/rails is not present
RAILS_GEM_VERSION = '2.3.5' unless defined? RAILS_GEM_VERSION
# Bootstrap the Rails environment, frameworks, and default configuration
require File.join(File.dirname(__FILE__), 'boot')
Rails::Initializer.run do |config|
# Settings in config/environments/* take precedence over those specified here.
# Application configuration should go into files in config/initializers
# -- all .rb files in that directory are automatically loaded.
# Add additional load paths for your own custom dirs
#config.load_paths += %W( #{RAILS_ROOT}/extras )
config.load_paths += %W( #{RAILS_ROOT}/public )
# Specify gems that this application depends on and have them installed with rake gems:install
# config.gem "bj"
# config.gem "hpricot", :version => '0.6', :source => "http://code.whytheluckystiff.net"
config.gem "sqlite3-ruby", :lib => "sqlite3"
# config.gem "aws-s3", :lib => "aws/s3"
# NOTE: jquery.js must be updated to the latest version 1.4.2
# to avoid differences in behaviour of HTML makeup and Rails
config.gem 'jrails'
config.gem 'nokogiri
config.gem 'uuid
config.gem 'rack-recaptcha', :lib => 'rack/recaptcha'
config.middleware.use Rack::Recaptcha, :public_key => 'my_secret', :private_key => 'my_secret', :paths => 'http://localhost:3001/my_controller'
# Only load the plugins named here, in the order given (default is alphabetical).
# :all can be used as a placeholder for all plugins not explicitly named
# config.plugins = [ :exception_notification, :ssl_requirement, :all ]
# Skip frameworks you're not going to use. To use Rails without a database,
# you must remove the Active Record framework.
# config.frameworks -= [ :active_record, :active_resource, :action_mailer ]
# Activate observers that should always be running
# config.active_record.observers = :cacher, :garbage_collector, :forum_observer
config.active_record.observers = :user_observer
# Set Time.zone default to the specified zone and make Active Record auto-convert to this zone.
# Run "rake -D time" for a list of tasks for finding time zone names.
config.time_zone = 'UTC'
# The default locale is :en and all translations from config/locales/*.rb,yml are auto loaded.
# config.i18n.load_path += Dir[Rails.root.join('my', 'locales', '*.{rb,yml}')]
config.i18n.default_locale = :ru
end

Answer 1

The Gemfile is a packaging element introduced in Rails 3. As you're running Rails 2 you need to do something slightly different. In you environment.rb add this line inside the Rails::Initializer.run block:
config.gem 'rack-recaptcha', :lib => 'rack/recaptcha'

Answer 2

I've figured it out.
I must add "require" statement for reCAPTCHA library