Google Adsense Page Level ads are a bit different than the standard ad units that get added to each post or widget area. The Page Level ad code needs to be added to your sites Header.
To do this,
go to Settings > AddToAll then select Header Options > Any other HTML.
Paste the Page Level ad code in the box and save your changes.
You'll also want to clear the cache to make sure you see the changes immediately.
Here is some additional information on Page Level Ads from Google.