mac is pretty unhelpful here in terms of actually mounting the external drives.
I asked at the last mac dev conference if there was a way to get a list of the drives that you see in the finder, but the experts couldn't think of one.
this means you need to mount the drive yourself. You can do this by manually browsing it.
at that point, it will show in /volumes, and VLC Streamer will list it as a known drive.
it isn't ideal - but I don't know a better way...