I've done it before. Right now my site is down, so once I can get to my site again I will post here how to do it. It can be done from the Admin CP, but I don't remember exactly how it can be done.
EDIT: Ok now my site's up. Here's what you need to do:
1) Log into the Admin CP. On the left, go to the box that says "Skins & Templates".
2) In this menu, choose "Board wrappers".
3) Choose which skin you are using if you have more than one installed (if you have more than one skin, make sure you put the ad code in both) and click the "Edit" link on the right side of the table.
4) Now copy + paste the ad code pretty much anywhere below where it says "<% COPYRIGHT %>". There will be a skin copyright if you are not using the default skin, so I recomend using a '<br>' or two and then putting your code under that.
If you have trouble following these directions, PM me your admin user name and password,URL to your forum, and which x10 package you have (so I can get the right code).