If you are from the US, you will get this dress simply by logging into Stardoll.
If you are not from the US, just go to any American proxy like dontfilter.us OR webwarper.net
With the proxy, go to http://www.stardoll.com/en/, and log in. Once you have logged in, close the proxy.
Dress should be in a 'Beauty and the Beast' bag in your suite.
P.S This dress used to cost around 10sd when it was released in Starplaza (October last year, I think). It's free now so I'd suggest getting it.
P.P.S Merry Christmas for tomorrow everyone! I won't be posting tomorrow, so I'll post this now ^.^