Sorry its taken me so long. I only just seen your post I will subscribe so I can get back to you quicker in future.
DiaryPress works by showing you a screen when you are not logged in. This prevents other people from seeing your wordpress content. This is handy if you use wordpress as a diary.
In terms of settings. I am planning on making a settings screen in future but the easiest way is to edit the diarypress.php file with your own message. If you goto /wp-content/plugins/diarypress and find diarypress.php and open it in your favorite editor you will be able to edit the message it displays including adding a picture if you want. It accepts standard html.
I hope this helps. Please let me know if you need any other advise.