If you have hit your requirements for the platinum requirements for 2013, you are eligible for the renewal gift.
In addition to Lucky's post here, I would like to add that in order to get the best use out of this, you should register as late as possible because once you register the 6 month expiration clock starts ticking.
Unless you have something you want to use it for soon, you can wait as late as January 2014. This was confirmed by Starwood here: http://www.flyertalk.com/forum/20860081-post284.html
Or read the entire discussion about it here
If you didn't get the email, you can register for the gift here.