I have also added the simple "exit" plugin in the game, but that also don´t work. I think something is going wrong when it is in IntelXDK.
I have followed this instructions:
1. You need to add the cranberry game plugin to construct2
2. Then add the plugin to your project and configure login etc.
3. Export to intel XDK
4. Ad your Google ID to intelxdk.config.additions
5. Enable the "Phonegap +Admob* Plugin" in Cordova settings of your project in IntelXDK (it is supposed to be activated right away but that is not working for me so I always enable that)
6. Set export option to "not signed"
7. Export via Android/Crosswalk
8. Sign your APK with your Key
9. Run app on a device
10. Get a cocktail to celebrate awesomeness
Number 6 I just set it to "signed" because I don´t know how I do number 8...