If the Mac is seeing a QuarkXPress document as a Unix file, simply add the three letter extension.
For QuarkXPress 3/4/5 that would be: .qxd (QuarkXPress Document)
For QuarkXPress 6/7/8 that would be: .qxp (QuarkXPress Project)
If you then double-click to open the document and it still does not open, then go into XPress and open the file via the File>Open command.
Yes, the resource fork may be stripped out. But adding the three letter extension forces OSX to recognize the file.