You can try refreshing your browser. I did this numerous times in order to get the nailpolish shade I wanted (granted, this was at 8am PST when most items were still in stock). Also, since this weekend is just the preview, they probably only have a limited number of products in stock for this event, if it shows up as not in stock now...then as tenngal mentioned, they probably won't be able to restock in time before the preview ends at Midnight tomorrow. Plus, the email clearly states that "limited quantities" will be available for this event.
Also, keep in mind that this event is just a preview, they will be relaunching the Full Hello Kitty Beauty line in mid-January on Sephora.com and in Full Sephora store locations. So if you can't order it this weekend, you'll only have to wait a month until it gets re-launched, so to everyone who wasn't able to purchase this weekend, don't worry too much, it'll all be back in stock for you to purchase soon... with higher stock and availability nationwide.
