In the Email Notification section of Preference page, if you want to be notified when someone leaves a message to you, select 'Yes' from drop down list, 'Send email notification to yourself when visitor sign my Guest Book'.

To send out Thank You email after your guests made a post, select 'Yes' from 'Send email notification to visitor who sign your Guest Book' drop down list.
Then fill in your email address that will entered as "<from>" in the notification email. Enter your customized Subject and Body of the email in the boxes provided.

Finally, scroll down to the bottom of the page, and click on the 'Save' button.