If you click on their names on the left hand side of the messages, you can then see when they were last online. If you post a reply on the thread, they may well receive a notification (depends if they have this option turned on or not) that a reply has been posted.
Otherwise you can send them a personal message (once you have made about 3 posts on the boards) by clicking on the small green scroll under their name.
I'll keep my fingers crossed that they respond.
Nell