With these it helps Apple does that, not you. You have to enable iAD Advertising on your app on ITunes Connect to receive adds. You either do that when submitting a new App or Update Application in the Managing Your Apps section.
may help you . Yes, you don't need to include ad code in onGUI() method. And also, you don't need to call LoadAd again and again. In my app build with unity, the ads do refresh. For example if there is no other ad available to show at that point of time, I guess the same ad will remain but as a new impression. Also, you can check the logcat while testing your app using Xcode ( for iOS) and Eclipse/Android Studio ( for android). It will show ad is refreshed.
AdMob interaction with bannerView.Destroy() and bannerView.Hide()
will be helpful for those in need In my game I use .Show() and .Destroy() on the banner every time the user passes a level. I would say its safe to do so. A tip: Make sure you are using test ads while testing your builds. About the banner not refreshing maybe its because of the targeting and google simply cant provide a different add. I'm sure that my ads refreshed with just the sample code from the ad mobs website.