Add this bundle (and its dependencies, if they are not already there) to your application's kernel:
// app/AppKernel.php
public function registerBundles()
{
return array(
// ...
new JMS\DiExtraBundle\JMSDiExtraBundle($this),
new Knp\Bundle\MarkdownBundle\KnpMarkdownBundle(),
new Liip\ImagineBundle\LiipImagineBundle(),
new Illarra\ContentBundle\IllarraContentBundle(),
// ...
);
}