Various games in the Mac App Store have in-app purchases (Pillars of Eternity I and II, Airport Madness 3D Volume 2, etc).
1) The purchase an In-App purchase item, launch the game from within your Applications folder.
2) You will now see the MacPlay launcher window. Here's an example of the Pillars of Eternity II screen:
3) Click on In-App Purchases. Here you can purchase DLC, etc. You will see a window that looks like this: