Recaptcha plugin problem

Hi all,

I am using radiant CMS and I am trying to create custom extension and
add recaptcha validation to it
but when I call
   if not verify_recaptcha(:private_key => 'aaaaaaaaaaa')
     #add error
   end
I get: undefined method `verify_recaptcha' for #<Comment:0x4f4c460>

Bellow is the full code for my model class

class Comment < ActiveRecord::Base

  validates_presence_of :title, :author, :comment, :message =>
'required'

  validate :check_captcha

  def check_captcha
    if not verify_recaptcha(:private_key => 'aaaaaaaaaaa')
      errors.add(:title, 'def error');
    end
  end

end

Please someone help

10x in adance,
Ilian Iliev