The text that appears when editing the Template is only a Sample. It's not possible to edit, and it serves only to show you where your real text will be. You will be using this template for various emails, so the text won't always be the same, right? You can add text when composing an email in your email server, GMail for example. Then click Preview to check if everything's okay.
But, on the other hand, if there is a certain text that you wish to add every time you send an email without having to write it every single time, you can use the plugin Text. Whatever you choose to write here will feature all your emails.