thats the "internal" (interior?) site thats why you cant get it, its for testers etc only... just wait for the main msn page (http://messenger.msn.com) to update
EDIT: USA is 5 hours behind me so it could be that if its released on the 22nd (today) it might be midday or so for USA, which is in about 2 and a half hours, so just wait and stop complaining that the int site wont let you get it