When a user clicks a button, I want a full screen iAd to appear, before the user can continue.
I can create the popup view, but how do I fill the whole view with an iAd? (e.g. like its done in wordfeud)
ADInterstitialAd (or full screen iAd) is only available for the iPad. You can't use it on the iPhone.
From iAd Programming Guide:
Full-Screen Advertisements are Only Available on iPad
Have a look at that guide to see how to implement full-screen iAds on the iPad.
Use the ADInterstitialAd class. You can present it from another view controller (e.g. your main game board’s) with -presentFromViewController:, or within an existing view with -presentInView:.