Rob: all I can say is, adding an existing folder works for me (Windows).
I can have previously created a folder, identify that in the browser window which opens from "Add Folder", and then have that folder appear in the LR Folders list (stating 0 images initially).
Alternatively, without having previously created this folder by other means, I can do in that in this same browser window (using the normal facilities provided there) and then select this newly made folder, with the same outcome.
Once this folder is visible inside LR, I suppose one could Sync Folder to show whatever images are stored inside it... although if one wanted to do that, some kind of Import would certainly have been a simpler choice in the first place, rather than Add Folder.
But the OP specifically did not want to Import anything.
RP