Posted with courtesy of "Community Managers" Group Site.

How To Post A Message,
Locate the message-board (some communities only allow you to select by topic)

you can either A: choose to post a reply to an existing message or B: start a new discussion

A
Locate the correct discussion. Click on it. Select Add Reply

A new window appears, Compose your message then click send. (the new window disappears and the message board refreshes your reply is now visible

B
Click start a new discussion

A new window appears, type in the subject for your message (keep it brief and to the point) Compose your message then click send. The new window disappears and the message board refreshes your reply is now visible.