Platform theme: Display an ad between posts

Hi there,

I’m new to wordpress, I’m using the platform theme and I would like to display a Google adword between post number 1 and 2.

With my previous blogging tool I have a php code close to this.

$pub = 1;

<?php if($pub == “3”){ ?>

<div style=”text-align:center”>
Google Code
</div>

$pub++;

I don’t know where I need to post this code, can someone help me with the code I need to write to post content between the posts listing ?

Thanks in advance.