You can click on the "inbox" symbol found on top of the page that's along the same bar as your "logout" link. Inbox --> New Msg --> Enter the Username you want to PM and Send.
Or, an easier method would be to click on "PM" symbol found under the member's post that you want to PM. It's part of every post. You'll see it attached to your own post aswell in this topic - Under your signature area.