Another comment is that you can still use Roon without an internet connection, however you will need to “periodically” connect to re-establish your license with the Roon servers. see this post: Roon with no internet - #9
The drag and drop instructions are for windows it appears.
I do not have a MAC so do not know if drag and drop works for MAC.
So, as others have said, if you have music files that you own that are stored locally on your MAC and you want them to be on/in the storage in your Nucleus+ the directions for accessing the internal storage on the Nucleus+ are:
"You can copy music to this drive by visiting the network share:
- Open Finder
- Press Command + K
- Enter smb://NUCLEUS/Data/Storage/InternalStorage
Be sure to replace NUCLEUS with NUCLEUSPLUS if you have a Nucleus+ If asked for a username and password, useGuestfor both."
Since I do not have a MAC I do not know any more than that, I would take screen shots if I could to help, maybe someone else that has a MAC could do that to show how this is done in a graphical way…