Deleted my first answer to improve it rather than posting more messages.
You will need to do a few of things.
First is to use FAT32 format on the USB memory stick. This will destroy all data on the memory stick if you have to reformat it. There are free tools out there for use on a Windows machine to format the device or you can use the B2 to format it. I do not know the toolset for the MAC. On the B2 these commands are under the Settings >> Maintainance menu. Be sure to select the correct letter based on where it is plugged in. If it is already FAT32, skip this step. When you initially plug in the memory stick it will report an error, just proceed with the format.
Add the FLAC files to the USB stick in accordance with the ARTIST/ALBUM/tracks file structure. You can see details at
Organising Your Music new (brennan.co.uk). If you were using Windows I would warn you not to let the computer 'fix' the memory stick. Doubt MAC has that warning.
Then you can do the IMPORT FROM C function, connect the USB stick to USB C.
Good idea to run SCAN DISK when complete.
You have a different option as well.
If you have a solid WiFi or ethernet connection to the B2 you could enable the NAS function, tell the MAC how to find it, and the following the same Organising Your Music link as above for the directory instruction, copy the files into the B2. Then you MUST run SCAN DISK. I can't give more detail again because I don't know the MAC at all. I add music in this manner on a regular basis. If you have more questions about using NAS instead, please ask.
Good idea to create a backup if you make changes that you don't want to recreate if something goes wrong. Always have a backup (or 2!).