I was a bit confused by that as well, once you're signed up and have installed the beta version, you still have to 'buy' pro as an in-app purchase, just make sure the payment screen says 'test card'.
Well i gave it a day and the test card part came up, but said 33.99/30mins. did a couple of things mainly soundknator down then cancelled the subscription within the 30mins, watch this space.
Edit, or if you read the email properly it tells you that you will not be charged, so lots of things to look through at the weekend at work