how to make guestbook link in menu?

Option 1: Copy the link of the front page of your template and put it in a href tag. The put that tag wherever your menu is enclosed.

Option 2:
1. Create a new page template in WP and name it guestbook.php
2. Go to the admin page and create a new page for your new guestbook template.
3. Open up guestbook.php and call an include(guestbook.php) statement for the front page of your RumWall guestbook.

This in effect puts your guestbook in your wp template (which you may have to modify your guestbook theme) and puts the link for your guestbook.