Hi Rich,
you have to "share" your iTunes directory. So locate first your iTunes directory (normally at "computer name"/music/iTunes/iTunes media). At preferences (which is normally in your dock) you go to "share" and there you enable data-sharing and you chose the itunes directory.
Now for the Unitiserve: go to Network -> Refresh. When you see your computer's name appear edit username and password and normally it starts scanning your files.
To play them you have to go to Browse music -> music location -> network" and there you will find them.
I don't have the Unitiserve, but I had one a few days ago borrowed from my dealer. I managed to access the files from my computer and NAS quite easily. So please don't mind if it's not 100% correct, because I'm writing this out of my memory.
I'm using German Mac Os, so the translation may not be exact.
Have fun! Tell me if you encounter problems.
Regards
Chris