Once you get to the Redeem step, you must be on version 2.1 or later of the IOS app. Otherwise you will not be able to redeem your purchase.
1
Open the URL
2
Select either the monthly or yearly plan
3
Select continue
4
Enter your payment information
5
Select continue
6
If you have the app, select "Redeem" otherwise download the app
7
This should now link the web subscription to your app