The first time you try to buy a program from the App Store, you’ll be asked to agree to new iTunes terms to govern your use of the App Store. When you do this, you’ll be delivered back to the App Store. Your purchase will not continue. Instead, find the program you want again and buy it again (you won’t be charged the first time; this slightly clunky mechanism is Apple’s standard method for this kind of change).
If the app is free, it will download right away. If it costs money, iTunes will process the transaction and then begin the download in the same way that standard iTunes Store purchases do.
