If plex has their own non-standard file sharing protocol, I don't think that Boxee should/will support it. The beauty of boxee, in my opinion, is that you don't need to have boxee running on your server. Just enable SMB sharing on your server and your done. The plex server is probably also hosting the plex database, which Boxee certainly won't read. Boxee just wants to see the raw files.
That said, I've found SMB serving from my Mac to be a bit flakey in Boxee. I think they are working on making it more stable.
Zotac ZBOX HD-ID11 w/ Next Gen ION
Windows 7 Home Premium 32 Bit
Intel Atom D510 1.66 GHz Dual Core
2GB DDR2 800
30GB Kingston V Series SSD
4TB External Storage