Great. You may have to tell MC to scan your Z drive and import any media it finds. Try this:
Options>
Library & Folders>
...configure auto-import...>
In the Folders section, click on the Add... button on the right.
Add z:\
Choose Mode: Include this folder and child folders
Tick off the media types you want MC to add to its library, e.g. audio and video
Explore the other options available. If you select run auto import in the background, any further media files you add to z:\ (your NAS), will be recognised and imported by MC.
When you finish and close the options, go to the top menu bar and go to:
Tools>
Import>
Select Run auto import now.
In a few seconds to minutes, all your media will appear and further additions will soon show up in the future too.