Here.
You need to have a MS Live account (the same one that's used for an MSN chat account, or for XBox Live). In fact, it takes you through the XBox Live signup, which was slightly confusing. You'd think they'd streamline that, or at least put different branding on it.
But for 10p, who can complain?
Edit: And after setting up my account, I popped back to the page and it referred to me as "WailedNeptune7", and I thought something had gone drastically wrong with their login system. But no, it had just assigned me a random "Gamertag", and then started referring to me by that name. I know MS are shit with usability, but really, What The Fuck?
