The beta is for advertisers, not publishers. Additionally, while the ads are contextually targeted like Google Adsense and YPN, they're currently only for display on Microsofts websites, not third party sites.
They have sent invitations to people outside of the US who currently buy advertising from them, but you cannot apply unless you received an invitation which is tied to the existing account. This is for people who want to purchase advertising, not publishers who want to run ads on their own sites as the thread starter stated.
I got an invite by mail. this is part of what it said why not try Microsoft adcenter today , it is easy to sign up and you can activate your new account for just $5, visit MSFTadCenter.com/dmj2A and be sure to enter the following code xxxxxxxx i hope this helps