This forum is designed to prevent casual visitors from downvoting other people's posts, so they've set up a threshold of 500 karma points before you're able to downvote. Your present karma is 26, so you have 474 points to go before you can retaliate against all those mean, unfair readers who downvote your posts. :)
Ah ok, thank you.
When people stop down-voting your comments or the quality is such that they start voting them up ;) then you will reach a certain score (changes over time) and magically you will have this awesome power to down-vote the comment contributions of others (please use sparingly).
And you.
There is a threshold of 500 (or it was, I suppose it could have changed) karma that a user must have before they have the option of down voting comments.