getting gravitar to work

I am trying to upload a theme I created to the wordpress free theme directory. Unfortunately, I need to have gravitars enabled on the thme before I can submit it.
I read this: http://codex.wordpress.org/Using_Gravatars#Theme_Support_for_WordPress_2.5

But I am a little clueless.

Do I have to add the if (function_exists(‘get_avatar’)) { (and the rest of that chunk of code) to my functions.php? When I add it, I get one of those php parse errors. Also, what exactly do I put in the comments template and where? I understand this:

<?php
echo get_avatar( $id_or_email, $size = ’96’, $default = ‘<path_to_url>’ );
?>

renders the little html gravitar image but where do I put it exactly in my comments.php?

Here is my comments.php by the way:
http://pastebin.com/9JhgcQKJ

Hope someone can help …