If the app is already published, click Yes to search for the app in the stores. If No, enter the name in Enter your app information and select the Platform.
Click Next: Create Ad Unit.
Choose from interstitial or banner.
Enter the name in Ad unit name.
Click Create Ad Unit.
Copy the ad unit ID to use later in the AdMob settings in the visual app builder.
Repeat the process as necessary for different platforms or ad types.