I too have a Macbook and a Lumia 640 and couldn't find a proper solution to this "sync problem".
Since I had a Windows 10 virtual machine that I use for Windows dev I also use it to access the Lumia files, but if you don't have one there isn't a solution atm, and I don't think Microsoft cares about it unfortunately...
Edit: I forgot to mention that Onedrive works wonderfully in OS X, so maybe you can try that. (For smaller files is OK, for larger files it will always depended on your upload speed :/)