Quick comment is how you add to an existing thread. If you notice at the bottom of the "quick reply" box, there's a "Go advanced" option. That will bring you to a page with a larger comment box and more editing options for your post. Or you can click on "Reply to Thread" below the last comment in a thread.
Pages are created automatically when a certain number of comments is reached.
You can create your own journal to post to daily if you want. Just start a new thread from the Primal Blueprint Journals page and then "reply to thread" or "quick reply" when you want to add a new post.