There are three ways to send a private message to a forum member. It helps to know that the term here is Conversation.
From a posting by the person to whom you want to send a message
Find a posting by the person you want to contact. Click that person's user name any of the places you see it. In the profile pop-up box that appears, click Start a Conversation. The user name of the person you want to contact will already be entered.
From your profile menu
Click on your user name at the upper right of any screen. Select Conversations, then click the Start a New Conversation box. In this case, you need to know the User Name of the person you want to contact.
From the intended recipient's profile
Click on the Members tab and enter the user name of the person you want to contact - again, you need to know the User Name or be able to enter enough of it that the system will suggest the name. On the screen that comes up, =>select the Information tab<= and click Start a Conversation.
Complete the message
Enter a title for your "conversation" and enter the text.
Your conversation will only be visible to the two of you, but you can always add people later to the conversation without having to forward all the messages to them. To enable the other person or people in the conversation to add others, tick the box that says "Allow anyone in the conversation to invite others".
There is Lock Conversation box (to have a one-way conversation?). Only tick that if it's appropriate.
You can add files and preview your note the same way as with postings.
To send your message, click the Start a Conversation box at the bottom.