Once in your back office select the profile tab in the top right corner.(This will be right next to your name and user TSA#) 
From there, select and fill out the data under the "billing" tab. (You can delete old cards by clicking the billing tab as well)

If you account is inactive:
You will need to click the subscription tab to reactivate. Make sure you select your payment method after clicking the "Reactivate" button.

Once the payment is made your account will become active again!