After you set up the Admob object (providing it with the necessary ID's), on the event sheet you'll need to preload the ad, and when it's ready, you have to display it. For example if you want to show a banner ad:
System -> On start of layout: Admob -> Preload banner ad
Admob -> On banner ad received: Admob -> Show banner ad
Keep in mind that the ads won't show in preview mode. You have to export it with Intel XDK to test the functionality (and you only have to set the Test mode to false in the object's properties when you are ready to publish your game, otherwise a test banner will show in the exported app).
I just learned this recently with the help of this awesome community. Indeed, the Admob manual page could use a better explanation.