What I have had to do to get it to work is - go to the person's posting on a page (or go to their profile and click on last post etc), then at the bottom on the left side, click on feedback score, then on the right hand side of the page, click leave feedback. Fill in Trade, Rating - then this is where I had problems - I couldn't not get it to work when I put in a link to the swap.
Then leave a comment.
If anyone has an easier way to leave feedback, please let us know!