The images below shows an approach to implement ads in App Inventor.
Please note that I have actually consolidated 2 apps into 1 so that I can show everything in one place.
In reality, it should be broken down into 2 as follows:
1. App1 would be an admin app that is used for managing ad campaigns
2. App2 would be an app that gets published to Market
Using the admin app, once you make update to your ads, all users-devices that has your app installed, App2 the one that utilizes ads, will display the updated ads. See attached file. It shows what needs to be moved to Admin app.
More info can be found by going HERE