Navigate to iTunes Connect > My Apps > Your App > Features (top tab) > Promo Codes > Generate
Redeeming Promo Codes
Don’t even dream of redeeming these on Apple TV. Just hop into iTunes or if you’re on iOS, into App Store, and redeem them as if you were working with an iOS app.
Then, return to Apple TV, open the App Store, and find the listing for the app in question (see previous post about how annoying this is Update: Or just use Robert Jen’s genius approach: Look under Purchases/Recent Purchases). It will be ready for you to install.