You can send a personal message to any member of the Forum.
Click on Members and select view the members list (you may have to search for them if the list gets very long).
Click on the member's name.
Their details com up.
Under their name part way down is 'Send PM'. This is send personal message.
Click on it and a posting box will appear. Write message and it will appear in your friend's 'my message' list.
Their answer will appear in your 'my message' list.
You can send a message directly by clicking on My messages and select send message.
In the To box you type your friend's visible username.