james'mommy,
To PM (Private Message) someone, click on their name in any post that they've made. A dropdown menu will appear. The second choice is "Send a private message to XYZ". Click on that and the page will change to the PM page. You can enter the text of your message in the white box, the same way you would enter a post to the boards. The difference is that the message will only go to the person that you're PMing and will not appear anywhere else on the DIS.
To read your own PMs, look for a yellow box in the upper right corner of any page that you're on. As long as you're logged in, you can access your PM mailbox. Click on the Your Private Messgaes link and it will take you to a mailbox which is similar to an email mailbox.