As of today, Music Mix is available to MSN Messenger 7.5 users as well as Windows Live Messenger 8.0 users. However the geographic restrictions still apply, it is only available to those users who live in USA, France, Germany, Japan and UK (or at least have their Passports set to those locations).
You can find Music Mix in the Applications menu. More info on Music Mix
PS: There's another Messenger post coming soon (ie within the next 24 hours or so). Watch this space!