Interesting. If you can see the Xbox in Beam, it seems like you've set up everything correctly. Make sure your firewall isn't block TwonkyBeam, it's control point or it's server.
There are some other suggestions here, if you scroll down in the thread. viewtopic.php?f=27&t=7697#p28350
In the meantime, we'll confirm that we can still beam to Xbox here. It's possible that MS made a change which breaks things. I'll let you know what we find later today.