You can place the PPD used in your Output Styles into the following locations:

For Mac:
Mac HD > Library > Printers > PPDs > Contents > Resources > en.lproj (the .lproj folder for your system language).

For Windows:

Once the PPDs are moved to this location, relaunch QuarkXPress Server. The PPDs will display in the PPD popup menu.

Note: See also Knowledge Base Article FE0044: 'No Option to Select a PPD Folder in QuarkXPress Server'.