You would have to login once the app is closed or if you do nothing in the app for ten minutes.
Regardless of how you get here, you will see the categories of Add-On you can buy.
Click on an Add- On Category to see which individual Add-Ons you can purchase.
Click the Add-On to expose a Buy- Now Button. Clicking it takes you to a Confirmation Screen.
If you click “Buy Now”, your Add-On will be purchased and your Balance reduced.