I had a few troubles getting Airplay to work under Mac OS 10.7. The solution is to disable IPv6 on your Mac :( .
To do this, you have to open the terminal and enter this:
networksetup -setv6off Ethernet networksetup -setv6off wi-fi
After you restart iTunes, it will finally be able to connect to your Boxee.
Sadly the Boxee guys don't want to fix this issue. Disabling IPv6 is a very drastic move :( .