Trying to add a Networked Drive to Plex

Hi.

I am a complete newbie to Ubuntu and Plex, which makes me a bit of a basket case. I am relying on the kindness of strangers :)

I have a Ubuntu 16.04 computer that has Plex Media Server installed. As part of the installation process, I attempted to give Plex the location of my iTunes library, my only real present source of movies and TV shows. The iTunes library is on a networked share that shares files between a few household macs and this Ubuntu machine.

The Ubuntu computer shows the drive as mounted in the Files file manager. I can access and open files on the same drive from Ubuntu [and the macs, of course].

But I have not been able to give Plex the location of the networked folder, despite the fact that the drive is mounted. I may be doing something wrong in Plex land or Ubuntu land.

Can someone steer me in the right direction?

Thanks.