Ad Mediation Plugins is using iAd implementation from the IOS Native plugins which is part of Stan's Assets products. In order to enable iAd Netwok you need to own IOS Native copy, which can be purchased from the Asset Store.
So set up steps would be:
- Import IOS Native plugin to your project.
- Setup iAd in iTunes connect
- Enable iAd Network in plugin settings as showed on the screenshot bellow.
Once you done with the Setup, you may use the iAd Network in your Mediation Profile.