There are three ways to do this. First the easy way: Click on the user id from any of the forum threads and click on "Send private message to...". The other way is to click on UserCP then click on "Send Private messages" from the Control Panel on the left, or alternately click on "Quick Links" from the top of the forum menu, select Private Messages, then click on "Send Private messages" from the Control Panel on the left. Click on Send a Private Message link on top of the forums page. You'll need to know the name of the member (the username/alias under which he or she posts) that you wish to send the PM to.