What I usually do is look to the top right of the screen. Youwill see something that says to the effect, "Logged in as xxx - xxxnew messages." Click on the "new messages" part of thatstatement and a text editor will come up. Type in theperson's username that you are trying to PM (it must be exactly as theyare registered; for example, you would have to typeBunnyMommy if you were trying to PM me, notbunnymommy; hope that's clear). Type in yourmessage, hit SEND, and you're good to go!
P.S. You can also just click onthat person's username to the left of their post. A menu willpop up with options for you. One of the options is "Send aPrivateMessage". Click on that, and the rest ofwhat I said above applies from that point.