One question to make sure you are in the right place: Are you a Free Trial or Existing user?
That's great! Welcome aboard. You might find this article on Converting Your Trial Account more helpful. (You are welcome to stick around and read this one too though if you like)
Excellent. You are in the right spot. Please read on.
Power Diary automatically processes your monthly fee and SMS charges using the credit card details stored in your account. You can edit your credit card details at any time. Here's how you do it: