Log in to Playground on either a Safari or Chrome window. Click on the Billing tab on the left hand side. Click the blue "Make payment" button to the right of your balance amount.

The Create a new payment window will open where you can select the payment method, amount you'd like to pay, and see a summary of the payment with processing fees. Once these sections are completed, click the blue "Charge $" in the bottom right corner. Once completed, you'll see a green success notification at the top of your screen.