1. Click on your profile in the top right of the dashboard and select Account and Billing from the drop-down box. 

2. Click on Add Credit in the top right of the Payment box. 

3. From here you can add credit to your account into the Amount box (Red Arrow). If you would like for money to be automatically added to your plan you can set a rule where if your balance gets lower than X-amount then Y-amount will automatically be added to your balance (blue and yellow arrows).