When you send a swap request you should see a link to "Make a not or add a review" next to the email sent line.
If you are logged in you can also add a note or review from their swap page.
It will be next to their name.
If all else fails you can use this link:
http://swapstick.com/swaps.nsf/UserReview?openform&reviewee=nobody&login
and change the name to whoever