Anyone has a page template for guestbook to share with me? Just simple as the ordinary post page. I cant find any single.php or page.php to modify. I'm a new user of this theme.

Found it myself:

Style & configure COMMENTS->Allow comments on "Page" pages, too? set to YES?

Then just write a page called guestbook with default page template.

If you have many pages you can close comment for the other pages individual in the WP editor ....