If you are wishing to start a conversation/private message another member of the forums, including staff, you can:
Click their avatar on a current active post which will bring up a small window with the option to "start a conversation":
Alternatively you can click YOUR name at the top of the forums and select "Conversations"
Then you can start a new conversation using the "Start a New Conversation" button at the top
Add the member's forum name in to the participant's space provided