You may have already seen the Nucleus Manual on the Roon Knowledge Base. Including it here for completeness.
This KB page should help you get music onto the internal drive. The “Copying Music to the Drive” section contains the network address that will let you shift files from your MacBook to the internal storage of the Nucleus.
To connect from your MacBook Pro:
In the Finder, choose Go > Connect to Server.
Type the network address for the internal storage of the Nucleus in the Server Address field. That address is:
smb://NUCLEUSPLUS/Data/Storage/InternalStorage
You should then see the internal storage of the Nucleus+ in the Shared section of the Finder sidebar and be able to copy and move files to it like a local drive.
In addition to @andybob’s directions: you should be able to see your Nucleus as a server under ‘Shared’ in your Finder window and browse to the InternalStorage folder like this (yours is probably named NucleusPlus instead of Rock):
There are various ways you can add storage to Roon (internal or external to your Core system, networked (NAS), etc.). It is up to the users to copy the files to the music folder that is defined in Roon settings – it is usually accessible as network share (like, in your case, the Internal Storage of your Nucleus Plus).
You will want to maintain a logical structure to your music files (Like Artist > Album > Tracks). Different providers of digital music have different delivery systems, but they usually end up in a single folder (like Artist_Albumname or something like that). In most cases, you will most likely want to rename this to Albumname and place it inside a corresponding Artist folder yourself.