Margaret,
A few questions which may add pertinent information to help you:
- are the files in question definitely in the formats which Roon supports; and not otherwise perhaps corrupt?
- can you see your Nucleus from its web interface?
- what happens when you restart your Nucleus?
- can you drag the files into the correct folder on your Nucleus directly from the MacBook Air itself (to do this, obviously you’ll have to copy them there from your external hard drive?
- is there any possibility that your Nucleus has little or no disk space left: you’ll probably need either to make it visible to you on the MacBook Air’s Finder or consult the ‘Internal Music Storage’ information via the Nucleus web interface (my suggestion #2)
Good luck with these pointers. Let us know how you get on and someone will take it from there 
.