ATP -- look to the left of every post. You will see the name of the member who submitted the post in the gray box. Click on the member's name and a drop-down list of options should appear. One of those options should be to send a private message.
Another option -- there is a medium gray box near the top right of the screen. It says "Welcome, ATP" -- in that box should be a link for private messages. Click on that, and then look on the left side. You will see a menu block called "Private Messages" and under that, and option to "Send New Message." Click that. If you start typing the member name in the recipient box, the search feature should display the similar member name choices in a pop-up box, and you can choose the right member name from the list.
NOTE -- membership levels have different amounts of allowed PM inbox/outbox messages. If someone's message box is full and they have not cleaned it out , they won't get your message.