It doesn't! We've realised there may be participants wholly unfamiliar with lj-based journalling sites, so we're in the process of writing a help guide for the confused. :)
Meanwhile, to post a prompt, click "Post a new comment" at the bottom of the prompt post. Please remember to read the rules first and fill in an appropriate subject line, then just type your prompt in the message box (you will probably be asked to solve a CAPTCHA before it posts).
If you'd like to fill a prompt, or comment on a fill, click "Reply to this" under their message on the prompt post.
no subject
Meanwhile, to post a prompt, click "Post a new comment" at the bottom of the prompt post. Please remember to read the rules first and fill in an appropriate subject line, then just type your prompt in the message box (you will probably be asked to solve a CAPTCHA before it posts).
If you'd like to fill a prompt, or comment on a fill, click "Reply to this" under their message on the prompt post.