Edit: please note the change to step 6... For me at least, com.rjfun.cordova.plugin.admob was not working..
Can also use FrostyElk AdmobFE plugin
1. get admob account
2. get and install AdmobFE plugin into C2 http://www.frostyelk.se/plugins/admobfe0009.c2addon
3. in your C2 project, add the AdmobFE plugin
4. In your C2 project, add appropriate code to show ads (see "helloAdmob" sample) using your ad ID(s) from AdMob
(4a Set AdmobFE plugin property "test Ads" to Yes when you want real ads or No for testing)
5. Export your project to Android
5. in your XDK project settings, under plugins and permissions|third-party plugins, click Get Plugin from the Web
6. enter com.rjfun.cordova.plugin.admob for name and plugin ID
6. enter com.google.admob for name and plugin ID
7. check "plugin is located in the apache cordova plugins registry"
8. click import button
9. build your app in XDK as Crosswalk for Android and install/run it on a device
10. observe your ads