Consolidate the iTunes library on your PC by clicking on "iTunes" at the top of the screen, and selecting "Preferences" from the drop-down menu. Click on "Advanced" at the top right-hand side of the window and make sure that both the "Keep iTunes Music Folder Organized," and "Copy files to iTunes media folder when adding to library" boxes are checked.
Make a note of the "iTunes music folder" location. Go to the "Advanced" menu and select "Consolidate Library."
Open iTunes on your Mac. Follow the previous two steps on your Mac's iTunes.
Go to the "iTunes Music Folder" saved on your PC, which you noted previously and transfer it to your Mac's "iTunes Music Folder." Transfer by either copying to an external hard drive or copy the files across a network and copy across directly.
Go to the "iTunes Library.itl" file on your PC by going to "My Documents" then "My Music" and then "iTunes Folder. Copy across to your Mac's "iTunes folder" and Delete the "iTunes Library" file on your Mac. Rename the recently transferd "iTunes Library.itl" to "iTunes Library".