The short of it-- to add posts, first register. Once logged in you can add a topic (discussion thread) to a forum, or post and comment to a thread in any forum.
To add a new forum (Main Theme) you need moderator or administrator privileges. Those privileges will be allocated by the forum administrators. You can request moderator privileges by sending an e-mail to jhall@plimoth.org . As a moderator you will be required to accept responsibility for monitoring and administering the forum at large.