I had trouble with this for days, too. I finally stumbled on the answer. On the main Activity page, it showed that I had an account balance in Paypal, but it said "Accept your money" under the balance amount. I then followed the steps to accept my money. Then I came back again create a shipping label. The calculate postage button and the review and pay button worked for the first time! I was able to buy and print the postage label.
