Do you have access to Quark 5? If you save the Quark Library back to Quark 5 from Quark 6, then from Quark 5, you can again save the Library as a Quark 4, which InDesign will open. There may be another way, but this method may be helpful in the event no else has a more immediate solution. You might check for a plug-in if you do not have access to Quark 5.