Update on April 17th, 2023: The Mac App Store has been redesigned since this tutorial was written. You can read an updated version of this tutorial for macOS Ventura here.
Sometimes, Mac developers are kind enough to give promo codes out so people can download a copy of their paid applications for free. A promo code is simply a combination of characters you can use on the Mac App Store to get the app for free, but how do you use them? It’s easy. First, open up the Mac App Store application. After the window has finished loading, you will see to your right a link that says Redeem. Click it.
After you click the link a sheet will pop up asking you to log in with your Apple ID to redeem your code, so go ahead and log in!
After you log in, you will be brought to a screen with a title that says “Redeem Code”. At the bottom, you will see a text field where you can enter your redeem code. Enter it and the application will start downloading! That’s it!