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.
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.
I think pg should let someone run Hacker News as their YC idea with the % equity for YC would being significantly increased. If the founder also released quarterly numbers and CPM rates we could all benefit from being part of the start-up in a meaningful way. This would achieve the intended benefit of allowing pg to focus more on the start-ups.