Admob is the Google Ad Network service for both Android and now iOS. It very simple to setup. You will need to create an Admob account http://www.google.com/ads/admob/.
Go to the monetize tab and create a new app entry if you don't already have one setup. You can create separate app entries per platform if you want to track things separately or you can just select a default platform and use the same app for both Android and iOS.
Next step is to create a default ad unit. Either an interstitial or banner ad. You will be able to setup additional ad units later on. An ad unit is one single ad. If you display an initial interstitial upon app load that would be one ad unit. Then you can display a banner ad on the menu screen which would be a separate ad unit.
Finding Your App Key & Ad Key
For each ad you want to setup in GameBuilder Studio you will need both your app key and ad key. The ad key tells the Admob plugin which ad to display on the Android device.
Admob Action Setup
Once you have the App key and Ad Key setup a Rules component in GameBuilder Studio and add an Admob action with the appropriate settings to match the Ad key settings you created on the Admob website.
You can respond to ad closes, ad clicks, and if an ad fails to display. If an ad fails try triggering a different ad action as a fallback (i.e. Chartboost, iAd, etc...). Whenever you select "Auto Preload Ads" the Admob action will pre-load the next ad to be show before the action actually triggers and right after a previous ad is closed.
**Note: Make sure to include quotes around the string key entries because the input field is a dynamic expression.
Make sure to deploy your game to an Android device to test that your setup is correct before deploying to an App store.