Originally Posted by PaperPrincess

Find a post from that person.
click on the number for their feedback score.
on the right side of the page you will find the option to leave feedback
The option is in blue - at the far right of the page "Submit Feedback for <member name>"

___OR____

Click on the Member List tab
Search on that member's username
when the name comes up, click on the number in her/his feedback score.