Doctrine Extensions Symfony2 - php

Question

I install Doctrine Extensions like here How to install Doctrine Extensions in a Symfony2 project
This is my Entity Class
<?php
namespace My\NewsBundle\Entity;
use Doctrine\ORM\Mapping as ORM;
use Gedmo\Mapping\Annotation as Gedmo;
/**
* News
*
* #ORM\Table()
* #ORM\Entity
*/
class News
{
/**
* #var integer
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* #ORM\Column(type="string", length=150)
*/
protected $title;
/**
* #ORM\Column(type="text")
*/
protected $content;
/**
* #ORM\Column(type="text")
*/
protected $description;
/**
* #ORM\Column(type="string", length=150)
*/
protected $keywords;
/**
* #Gedmo\Slug(fields={"title"})
* #ORM\Column(length=128, unique=true)
*/
private $slug;
/**
* #Gedmo\Timestampable(on="create")
* #ORM\Column(name="created_at", type="datetime")
*/
private $created_at;
/**
* #ORM\Column(name="updated_at", type="datetime")
* #Gedmo\Timestampable(on="update")
*/
private $updated_at;
/**
* Get id
*
* #return integer
*/
public function getId()
{
return $this->id;
}
}
php app/console doctrine:generate:entities My
gives me a a error
[Doctrine\Common\Annotations\AnnotationException]
[Semantical Error] The annotation "#Gedmo\Mapping\Annotation\Slug" in property My\NewsBundle\Entity\News::$slug does not exist, or could not be auto -loaded.
Should i add something in config or someting else? Thanks for help.