I was able to use this but you'd need to have a different Facebook account if you've redeemed it in the past (from the same account). It appears that it does not matter what email address you use to acquire the code, it will work on an existing Zinio account.
For the individual that had a problem in the past, I did as well and I think that was a hiccup on Zinio's side. I tried a second time (during the original promotion) and received a confirmation via email.
For this go round, this is what you'll see upon redemption:
Your voucher allows you to choose any 6 mini-subscriptions or 18 single subscriptions that you would like until 2013-04-19.