In previous version, Place holder pallet works while importing a XML file in QuarkXPress. QuarkXPress 10 does not have the functionality to import the XML file. This functionality has been removed from QuarkXPress 10, and this is the reason you do not see the place holder pallet in QuarkXPress 10.
If you want to import the XML file, you have to work with QuarkXPress 9. QuarkXPress 9.x has the functionality to import the XML file.