warlock quest the void summoner the choker does not summon the void walker

You must activate the choker in the it's circle.
This circle can be found in the slaughtered lamb.
In the room with all the trainers, you will find a small doorway, go through it and search the summoning circle in the 'basement'.
Once standing there, activate the choker, slay the VW and voila.
Here's a pic of the circle

This information was addapted from wowhead.com.
Learn how to use it
If this does not work, then it's a bug or some mangos crap.

I can confirm this quest works on the Alliance side.