I recently got a new mac, and wanted to use the iTunes music folder I had been using with the old one.. The iTunes Music folder is on a firewire drive, so I plugged that in, and selected it as the new library folder in itunes preferences. Nothing happened.

I thought at first it was a permissions thing, but it turned out, it was a matter of importing the old library listing.

The solution:

You need to have access to your old home drive on the old computer.

Open iTunes on the new computer select File: Import.

Then Users/username/Music/iTunes/iTunes Music Library.xml

Then the entire library will import.

See Apple TechNote on this topic