Currently, Google Admob is only available on Windows Phone 8, Android and ios.
I believe that Google Admob and other advertising providers will make their SDKs for Windows 8 applications as soon as the platform gets more success.
Until then, you can use one or the following combination:
Microsoft PubCenter
Ad Duplex
Leadbolt
scoreoid
Mediabrix
More here
You can also use AdRotator to achieve a fill rate of + 99% when exchanging / replacing real-time ad providers.