1. Add the admob plugin to your cocoon project.
<cocoon:plugin name="cocoon-plugin-ads-ios-admob"/>
<cocoon:plugin name="cocoon-plugin-ads-android-admob"/>

2. Add cordova.js to your index.html above your other scripts.
<script type="text/javascript" src="cordova.js"></script>

<script type="text/javascript" src="lib/impact/impact.js"></script>
<script type="text/javascript" src="lib/game/main.js"></script>

3. Put this in the init function of your title screen, for example.
//////////////////////////////////////////////////
if ( ig.interstitial ) {
    ig.interstitial.load();
} else {
    ig.interstitial = Cocoon.Ad.AdMob.createInterstitial("ca-app-pub-XXXXXXXXXXXXXXXX/NNNNNNNNNN");
    ig.interstitial.on("load", function(){
        ig.interstitial.show();
    });
    ig.interstitial.load();
}
//////////////////////////////////////////////////

Now, each time on the titlescreen the ad will called.