Do the standard tutorial in the manual, under 'admob ads' plugin. It does work.
1. Get an admob account (i.e. follow the 'setting up' steps in the manual, under 'Admob Ads ')
2. Make your game
3. Add ads to your game:
3a. Add an Action <On start of layout> -> AdmobAds:Preload interstitial (if you want a full page ad)
3b. Add an Action <whenever you want full page ad to appear> -> AdmobAds: Show interstitial
3c. Add an Action <whenever you want banner ad to appear> -> AdmobAds: Show banner ad at position <whatever you choose>
4. Put in your AdmobAds plugin properties, your Ad ID's for Banner/Interstitial. Set Test mode to Yes.
5. Export your game from C2 (follow steps 'Exporting' in the manual)
6. Build your game using Android Crosswalk in the XDK
7. Download the APK from the email sent to you from the XDK
8. Test your game by installing the APK directly
9. Set Test mode to No; re-export/build; publish to the Play store