Ah... You see, that only works if you are using a shooty weapon, no exceptions. I assume by the tempest signet you are under level 10. You should be able to get a shooty/staffy weapon called Snakebite Set, which you can get from the following.
I think the reason could be that you need a Melee weapon (ie Slashy, Stabby, or Smashy or a staff with one of those) to be able to use most Critical Strikes. I read recently in the Message Boards about a Critical Strike for Witchdoctors. I think it was called "Intuition". If this isn't it, then one should be created especially for the Witchdoctor's attacks.