If you are from USA, you will see the advertisement of the dresses at the front page, just click on the dresses or at the blue tag that says FREE GIFTS and you will be redirected to a Monster High Prom page, now you can close the tab and the dresses will be at your suite.
Now, if you are from anywhere else, you need to use a proxy.
You also can get the dresses using webproxy, use one of thesehttp://dontfilter.us/ or proxite.us/
Paste this into the proxy url bar: www.stardoll.com
Hit enter and log into stardoll.
Type this into the proxy URL:
You will be redirected to the Monster High Prom page, then return to the Stardoll page (click the arrow to return to Stardoll with the proxy, ok?)
Now, type this into the proxy's URL to logout from your account:
Done. The freebies will be at your suite in a while.