The error message 'A valid copy of QuarkXPress is already installed on this machine.' can be resolved by trying below steps:

  1. Close error message pop-up
  2. Quit QuarkXPress application.
  3. Navigated to Mac HDD/Users/Home User /Library/Application Support/Quark/QuarkXPress
  4. Trash com.quark.quarkxpress 9.setup.plist file
  5. Re-install QuarkXPress application.

If you have further issues please feel free to contact http://support.quark.com