First thought would be that you're attempting to use a z600 driver with a z715 printer. I have a z515 myself, but my assumption would be that the z600 driver works with all 'previous' printers of the z family, and my personal opinion is that any z driver should suffice for any z family printer, unfortunately Lexmark isn't as yet that thorough in offering drivers for all of their printer models.
Also, I had the same problem finding the driver on my harddrive, so try looking in /usr/share/cups/model after you complete the tutorial. In that folder there will be a .tar.gz with the name Lexmark-z600-lxz600cj.cups somethingorother. Right click and extract it to that folder and it will turn it into a .ppd. That's the driver you use in the printer setup.
Hope that helps. I'm a newbie myself, so I have no problem batting back and forth with solutions.
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