If you have a Google Site and use Adsense you probably saw this announcement.
Starting October 10, 2013, it will no longer be possible to modify AdSense ad code on Google Sites pages.
It also says this
However, any ads you’ve previously placed on Google Sites will continue to show, and you’ll be credited for all valid earnings.
This method will allow you to include ads on your sites using a gadget mechanism similar to one used for legacy ads still being shown.
You'll need the ID of the ad unit you want to show. You'll also need the publisher id which you'll find in the account settings of your adsense account. it looks something like ca-pub-xxxxxxxx
You'll find it at https://storage.googleapis.com/goinggas.com/public/hosting/sites/xliberation/xml/sitesense.xml
You just need your ad unit ID of the ad format you want to show, and the publisher ID. The width and height doesn't need to match the ad format, but better if it does
And that's it- you'll be displaying ads like the ones below
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