We make changes on your themes functions.php. If your theme have no functions.php simply create one and paste this code in your file ;

Here is the Code;

<?php
 function insertAds($content) {
 $content = $content.'<hr /><a href="http://www.wpfunc.com">Hola, Awesome, Quick, Simple WordPress Functions</a><hr />';
 return $content;
 }
 add_filter('the_excerpt_rss', 'insertAds');
 add_filter('the_content_rss', 'insertAds');
?>

If you change $content area (between <hr/> and <hr/>) you can change it with your google adsense code.

That’s All! Have a nice day!