Purchases for The Walking Dead Life are not currently available on iOS.
Facebook does not make the option to purchase available through messenger.com versions of games. To make a purchase you must access the game through facebook.com or the Facebook app instead.
You can access the Facebook.com version of The Walking Dead Life directly at the following link, which should give you the option to make a purchase: https://www.facebook.com/gaming/play/332716524362573/
ImportantIf you are entering the game through a Messenger notification, then you will be unable to make a purchase until you launch the game from the Facebook app or via the mobile web version of the game.