In the output pages tab, go to the output page you want to edit.
The easiest way is to use the "source" button and add it directly to the HTML code.
Once you see the code, scroll all the way to the bottom of all the code and add this (but obviously put in your own footer content):
<footer>
<p> Here's my footer content... </p>
</footer>
So it will look like this:
