ok as an example signup at Admob.com and in Monetize setup a New ad unit as a Banner. go into Settings > Advertisements and enter your Ad unti ID in the Admob Banner box. Then in Menu Editor select Game Field then select AdMob from the drop-down for the Ad Banner. your Ad should now be displayed as a banner at the bottom of the screen whilst playing the game ON YOUR DEVICE.
As an example: Add a Purchase Button to a screen in Menu Editor and select the Purchase Method and Ad Network as Rewarded Videos and HeyZap.