Yeah I've discovered Windows 7 networking to be a royal PITA! They did something to it that stopped my media streamer from working, hence why I bought the LS dou in the first place. Now I find similar problems to what you are having. I can map the LS OK but I can't see the NAS with the client utility.
If I plug my Vista drive back in then it all works perfectly. Both vista and W7 are 64 bit versions, all other software is identical.
(Edit: I set the IP of the nAS to 'static' and it now gets detected in W7 )