Interesting, I wrote Apple Support yesterday inquiring about the whereabouts of my key, and they emailed me this morning with the following directions:
1) Go to the Mac App Store.
2) Sign in with your Apple ID and password.
3) Under the Quick Links section in the upper-right corner, click the Redeem link.
4) On the Redeem page, enter your content code and click the Redeem button.
Unfortunately however, I can't find the "Quick Links" section in my Mac App Store window.
Anybody knows what's going on?