Actually, there's surprisingly little difference between the app area of iTunes Store and the Mac App Store. To the point where it's a bit odd that they're separate apps, but perhaps that's to avoid us users getting confused for something we buy for our iPad or iPhone and something we purchase to run on our iMac or MacBook Pro, etc. Silly users. :-)

A redemption code is going to look like this: LWKMW3YM99NR (that one's already used before you get excited and type it into the Mac App Store, btw)

Once you enter the code, the app knows what it's associated with and what to do. Let me show you...

First off, launch "App Store" from the Apple Menu on your Mac OS X system. On the home page look for the Quick Links box on the right side:

You can see the third choice is "Redeem". If you don't see "Welcome" as the first one, btw, you might need to sign in to your Apple account. If not, it'll prompt you shortly. Click on "Redeem" and here's what you'll see:

Well that's easy. Enter your code...

Click the "Redeem" button after you've double checked that you have entered the code correctly and it'll probably prompt you to log in to your account again. A typical prompt window you've probably already seen a thousand times:

If you've entered the gift card or download code correctly, you'll now see a confirmation that it worked:

Interestingly, it doesn't say what you're actually downloading, nor does it show a progress bar. It's all hidden behind the scenes. Unless you know the trick...

To see how it's going and what you're actually downloading, click on "Purchases" along the top of the Mac App Store window. In this case, we learn what I'm downloading:

The latest version of the terrific iCal replacement app BusyCal. You can see it's 8.6MB in size and that I'm about half-way through the download at this point. Once it's done, there's no message, sound or any other indication. It's just... done.

That's it. Now, go grab that game from your friend and have fun with it!