The initial release of the update for iOS inadvertently disabled the Tianxia product in the App Store, which resulted in a disabled Add-ons menu option in the app. This should be corrected now, but you may have to kill and restart the app to get it to request the product list again. To do so, double-tap the home button and flick the app to remove it from the running app carousel.
If that doesn't help, also check that purchasing isn't disabled under Settings->General->Restrictions on your device.
If you still can't get it to work, post a message here or send an email to firstname.lastname@example.org and we'll work with you to try to figure out what else may be causing the problem.