This plugin has not been updated by its developer since a long time ago.
It could imply partial or total lack of compatibility with the latest versions of b2evolution.
This hack lets you use a post as a guestbook that you can link in your sidebar or, if you prefer, in your bloglist. Normally a post will go off your main page as you post more, and, depending on your setup, you might not allow comments on posts. With this hack your 'guestbook' post link stays visible AND comments get opened if they happen to be closed or disabled. It gives your visitors a way to just say hi or comment on your blog in general without leaving a comment on a specific post.