If you are on another person's personal page you can click "Talk" on the left (or the "talk" link by them in the history of a page).
To leave a message for someone it is easiest to type your message into the new built-in field. Of course you can always add a message by simply editing the page and adding your message to the bottom.
When an edit is made to your talk page you will be notified with a message alert across the top of AboutUs.org.
For more information