[xenoterracide]

you need more karma. I'm not sure but you may have to have more karma than the person you are wanting to down vote. The only way to get karma is to post.

[breily]

You can downvote someone if you have less karma, but you have to have at least 25 (I could be a little off on the exact number). Nobody can downvote submissions though.

[izaidi]

I think the threshold is 20.

[Prrometheus]

I can down-vote some people but not others.