This is happening because of blocking. Your guestbook is on an unsecured folder of your website, whilst trying to display in a secured page on your website. You should ask your host to ensure that the HTTPS is applied to all subfolders of your domain. They can do this by making a change to your htaccess file.
It may be that you may have added the website address to the script somewhere either during installation or somewhere in the back-end admin area. If so, you should ensure that you alway use the HTTPS designation in the URL. Although the script itself will still work, it will pull the script from the non-secure URL, which is probably what is causing the issue. If you don’t want to reinstall the script, just ask your host to override the block through htaccess. You can see the problem in the source code of the page:
<iframe src="http://www.yumchamama.com/guestbook/preview.php" frameborder="0" scrolling="auto" width="100%" height="1200"></iframe>