There are several ways to do this. Here are a few.
You can click on their name, within the answer and go to their profile, on that page:
On the right hand side - you can type in a note on their public bulletin board.
On the left hand side, under their details you can click [send private message] and write them a little note...
You can click comment, under their answer, and type in a thanks and it will post with the Q&A..
I am sure any of these would be appreciated!