Same problem here... Don't know if it's a problem with the "Store" app or if the feature just isn't there yet.
Basically, I can't use my Xbox Music Pass at all on my phone.
Re: Official Windows Phone 8.1 Update 1 Discussion nd Problem Solving Thread
Don't know if its related or not, but I can't play any YouTube videos at all since installing 8.1 Update.
Every video throws a "Decode Error" message.