Hey @ben curtis 

Where exactly are you trying to book?
Are there any error messages?

Actually, it should work very simply this way:
Open deezer.com in the browser
log in with the account data
then you should see something like this

click on resubscribe, then you will be directed to the offer wall and you will see different subscription models, e.g. Premium:

Click on subscribe and you will get to the page with the different payment options:

After selecting your payment method, you have to confirm the booking and can then use deezer to the fullest.