Adding Adsense between blog posts

Hey,
I’m trying to hard code an Adsense banner into my word press blog, but I can’t seem to figure it out. I know that I should add

<?php if ($count == 1) : ?>
AdSense Code
<?php endif; $count++; ?>

right after
<div class="entry">
in my index.php, but there’s no div class=”entry” to be found anywhere. I do have a div class called “entry-content”, but it doesn’t seem to work after I insert the code below that class.

Any help is appreciated!