Go to 'MemberList'.
It will list all members usernames, and last visit date.
Find the user you wish to contact.
Look to the right of their username- Across the screen, you will see very bright lettering (pretty hard to make out what it says). The glowing letters say "PM" (Private Message). If you click the glowing letters, it will take you to the page to send the message =)
Another way to PM someone, is under their posts, it will show 3 glowy words.
The middle glowy word says PM and will take you to the same page as the other PM button. =)
Feel free to practice by sending me PM's (I get bored xD).