I'm having the exact same problem. I've read the modding guide and followed the instructions as written. But on mac the Improved Portraits mod does not fetch the portraits  from any pack you install with it. Even when manually trying to set a portrait under the customization and select folders menu to a folder where I've downloaded the pack, the path is incorrect. The window that opens up is way too small to see a list of folders so I have to manually open the directory in another folder outside of game, right click to get the path name, and copy and paste it into the path box under Customization->Select Folders->Portrait folder. When I do this, the folder path does not display correctly. Instead it just reverts to user://Applications - which is not the correct path. So mods currently cannot load on Mac and you can't set a manual folder either. Can anyone help on this? 

Thanks