I've added the ability for all users to send private  messages to other users, when you sign in the forums there will be a new tab at the top of the page called "Inbox" click on that it will show you any messages you've sent and messages other users have sent to you.
To send a new message click on"Inbox" then "Start a New Conversation" then in the box under "Recipients" start typing the user name of the person and a drop down will appear with similar user names. Type your message and click "Start Conversation"