If you have recieved this message already, skip the first step.
Go into the clockface settings, and select "End Trial Now"
Once you have selected this, you will see a screen like this:
After you see this screen, visit the link it directs you to visit, and enter the code displayed.
After you have entered the code, press "continue" and enter your email and payment details.
When you are done,you should see a screen like this in Fitbit Settings:
Your product is now licensed. Thank You for your support.