how do i go about to add a rep or - rep. I have no idea
Go to a user's profile and click "Rate".
One question , is it possible to rate from user posts?
And if not , is it possible from plugin ?
And can user rep other users?
Yes, users can rep other users if you say they can in the group permissions, that's the main point of the system really. And yes, if you click the rep count in the postbit, you'll see the rep page, then click the Rate User button.