If you are reading a message on a board and you would like to send the poster a private message, there's an easy way to do that. 

Step 1.  Click the green down arrow next to the poster's name to open the drop-down menu.

Step 2. Select Send PM from the drop-down menu:

This will open the Private Message composition box with the member's user name already in the "To" field. 

Step 3: Add a subject line, compose your message (and don't forget to use the Rich Text Editor tools), then click Send Message.