For the Advanced plan:
You'll need to edit the footer.php that is located in the "wp-content/themes/YOURTHEME" (Where I put YOURTHEME, it should be the theme you're using). You can edit the file by using an FTP client, or FileManager (from cPanel). Open the footer.php and add the Advanced Ad code at the very top. And that's it, save the file and you'll have the ad working.
NOTE FOR THE ADVANCED PLAN: To make the ad look nicer, you can add the following code to center it:
Code:
<div align="center">ADVANCED AD CODE HERE</div>