Edit and/or add Field names at adentry.php

How can I add and/or edit fields on the "sign the guestbook" page? I see it's adentry.php but I don not see this page on my templates list.
Thank you,
Rick.

I am playing around with some things from previous posts and think I am missing - or do not understand - something basic: Once I open a file that contains text I want to edit, can I just edit it like I would html?
Thank you,
Rick.

Here's an example: I want to personalize the text that currently says "Your Message" on the "sign the guestbook" page. I cannot find the file where I would edit it...can someone point me to it?
Thank you.
Rick.

Go into the Lang folder and edit the file called english.php
Go to line 43 ($LANG["FormMessage"] = "Your Message"

and change it to whatever you want.

Super!
thank you very much!
Rick.