Read original post by Lee Stott at MSDN UK Team Blog
We released a new version of the Ad SDK during //Build 2013 Greetings from BUILD! Announcing New Tools to Help Developers Boost Monetization in Windows 8
We have a new version for 8.0 and another for 8.1 that is now included in Visual Studio 2013. The new SDK can be obtained at http://adsinapps.microsoft.com/
In summary, the new version now enables targeting of Windows 8 and Windows 8.1 and supports changes in the available ad sizes.