Open your Address Book and find the person you want to send email to. On that persons's card, single-click on the gray label that's to the left of the email address. A menu pops up, as shown below. Choose "Send Email," and a new message window appears with that person's address in the "To" field.
Also see page 411 about using the Address Pane while in the Mail program.
Make sure the Edit button is NOT depressed or you will get different pop-up menus!