iTunes Library.itl (without the extension on OS X). When iTunes quits, it also writes all of its library information to the file called iTunes Music Library.xml. TuneSync transfers information between these XML files.These XML files are always written to your user folder. In order to use TuneSync, you'll have to be able to find them.
{MyDocuments}\Music\iTunes\iTunes Music Library.xml~/Music/iTunes/iTunes Music Library.xml