To include Sonata MediaBundle in the Admin Bundle, you'll need to add for example an Image field to your Article entity.
/**
* @ORM\ManyToOne(targetEntity="Application\Sonata\MediaBundle\Entity\Gallery")
* @ORM\JoinColumns({
* @ORM\JoinColumn(name="image", referencedColumnName="id")
* })
*/
private $image;
And then refer to it in your ArclicleAdmin :
->add('image', 'sonata_type_model_list', array('required' => false))