Hulu has blocked Boxee from accessing their content, but you can use this plug-in to get around that.
XBMC has already put together a quick plug-in to bring Hulu back to both XBMC and Boxee. A Team developer called d4rk at XBMC has created a plug-in to the Hulu plug-in which does some fancy decrypting to get Hulu working on Boxee and XBMC.
Hulu Plugin for XBMC [XBMC Community Forums]
How To View Hulu In Boxee [Boxee Forum]