When you see a post you like and want to add to someones rep then just go one of their posts in the thread. Under the name you'll see a bunch of little icons. Click on the second one that looks like scales, that should be the add to reputation icon. Then this should pop up

What do you think of whoever's post?
I approve
I disapprove
Your comments on this post:

Then just click on add to reputation. And voila! I think I got it right anyway. It's not a much used function, but I think it's a nice way to give a little pat on the back to those who contribute so much!