Interstatial Admob using PhoneGap not working

I'm trying to display interstitial Ad in my PhoneGap app but it's not working; it's not displaying the ad at all. I followed the instructions here.

Apart from trying to make it work, what I also need to do is to show the interstitial at a particular time (probably when the game is over) but I dont know how to refer to the Java code from JavaScript code to show the loaded interstitial ad.

Please help.