acts_as_tree error

I am getting the following error when I attempt to use acts_as_tree. I
am new to this relationship and am stuck.

undefined method `acts_as_tree'

Model
class Rental < ActiveRecord::Base
  acts_as_tree :order => "sort_order"

  validates_presence_of :name
  validates_numericality_of :sort_order
end

Schema
create_table "rentals", :force => true do |t|
    t.string "name"
    t.integer "sort_order"
    t.integer "parent_id"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

What am I missing?

did you install the acts_as_tree plugin ?

You have installed the acts_as_tree plugin, right?

   script/plugin install acts_as_tree

(it's on git://github.com/rails/acts_as_tree.git)

-Rob

script/plugin install acts_as_tree
(git://github.com/rails/acts_as_tree.git)

I didn't realize it was a plugin. The articles and book I was
following led me to believe it was a native relationship.

Thanks!