Once you have the AdSense code you can add it to Mediawiki following these steps:

1) Open skins/Monobook.php and get acquainted with the syntax for the different sections. This is the place to add your AdSense code.

For example, to add the google adsense code at the bottom of the pages, add the code right after:

                // Generate additional footer links
?>

After you make the modifications you need to open and resave your LocalSettings.php file. This will clear MediaWiki's parsercache and your changes will show correctly.