In your Reactor app, you can display iAds in your iOS build and AdMob in your Android build. The ad support settings are located in Build/Test tab for each App you have. Scroll down to "iAd" and "AdMob" settings. Check iAd to display ads on iOS and/or add an AdMob ad unit id for displaying ads on Android.
AdMob ads are presented by Google. You can visit AdMob and create an account. When creating an ad unit, choose the banner type.
After saving the banner ad you will receive an ad unit id. This is the id you use in the reactor settings.