I saw a blog post by tap tap tap a while back with a way to create user friendly links for redeeming iOS app promo codes. This is so much easier for the user than sending them a code and giving them precise instructions on the way to redeem it, especially if they've never done it before. You can send a link and they can get a free version of your app with one-click.
I hadn't come across a way to do the same thing for the Mac App Store until yesterday, when I was downloading the Mountain Lion preview and saw how Apple did it. You can use the following format to send out promo code links for your MAS apps:
Obviously, replace YOURPROMOCODE with your promo code. If you don't include the "?code=" portion, it will just open the redeem code landing page, which is pretty handy as well. Digging a little deeper, it seems the Mac App Store has two different URL schemes: "macappstore:" and "macappstores:". The latter—with a trailing s—is for secure URLs. If you're trying to direct link to something in the Mac App Store, and you get a "Cannot Connect to the App Store" error, try it with "macappstores:" instead.