you are using v 1.4 are you? (i would recommend upgrading) but if you don't want to.
install phpmyadmin from
www.phpwizard.com (i think that is right, otherwise search for it at
www.hotscripts.com
Then once installed (it is very useful) click on your database with your forum in it and then you should see all the tables appear. Click on the privatemessage one and copy and paste the next page. It just shows me what the names of the columns are.