You have until June 30, 2012 to register for this promotion. I don’t see this being particuarly good unless you plan on staying a few times this summer at SPG hotels, but a promo nonetheless.
You choose one of three dates, the type of reward, and one way to earn them. Obviously you’ll want to pick the latest dates unless you have some recent past stays you can get credit for since it’s retroactive. The easiest is 500 points for a stay at Sheraton. If you’re going to stay at least twice though I’d probably choose the double starpoints option. Just make sure you pick the one you think you’ll be able to get, since you can’t change it later on.
Here’s a graphic of your choices.