I've never been able to make live365 play for a very long time. There are ads (I assume) unless you are a VIP member. In any case, there always seems to be some sort of announcement about the advantages of a VIP membership.
To be honest, I would recommend Radio Time, which really seems to work quite well with Media Center and the 750 as an extender. The radio stations maintained by this site are always free of charge.
Check it out at: http://radiotime.com/