If you are using the AdMob FE plugin, then you will get an example file with the download.
There are two simple steps for an INTERSTITIAL ad to show up.
1. Create ad (This tells AdMob to produce an ad for you. When AdMob has made an ad, your app will download it and cache it. Preferably you do this on Start up, then you need to give it a few seconds to load before you show it.)
2. Show ad
Best case is to create banner at start up, then show it once the level ends or you can wait for it to be ready and automatically show once its ready to be shown. But that will interfere with the game and its not something i would recommend.