First of all, this should be in the Bugs and Features forums.
Anyway, at the right part of every post there are 3 images, report, reputation and post's number. The middle one is reputation, approve to give reputation, disapprove to take reputation.
Did you bother reading Jindo's sticky or use search?
Really, that the least you could do before asking!
Note that when having negative reputation you can give nor take reputation, you gotta get to at least 0 reputation points.
I do know this - xenforo dropped the ball by not keeping the vbulletin reputation comments as a feature. The loss of the Reputation comments data when we switched to Xenforo really was the death knell for the site when it came to all the users that left. I know I missed it so much and I got way less interested in the site when that feature was gone and I run the site.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory