You need a few points yourself first (no idea what the threshold is). Post some interesting items or join in the debates in the "comments".

I think you can probably see that this is a guard against "sock puppets" being used to promote spam posts.

The threshold is about 2 karma points for being able to upvote a story.

It looks like on this site, anyone can express an opinion of agreement with an up vote but only some who have earned it are allowed to express they disagree by using a down vote. Anyone can comment though.

No matter how many karma points you have, you can't downvote a story. You can only upvote or flag a story.

People do sometimes get their comments downvoted so I'm not sure what you are saying?

I was talking about a link that you submit cu HN. You can't downvote that, the original question was related to a story (or a link) submitted to HN.

I believe at 500 points you can downvote comments.

Thank you all. It works.