With the 645m the device is in "MTP mode" instead of Mass Storage. I have windows, so can explain how I do it there, and someone with a mac can give details about that. (but I think you need a 3rd party app like Android File Transfer.)
Ok, on windows 10, here's what I do: In the "build for device wizard", you can't access the 645m, so I build the .prg to my local hard drive (something like c:\garmin\apps, so it's only a minor change from d:\garmin\apps which is what works for me with other devices).
So what I do then is go to c:\garmin\apps, and copy the .prg, and switch to the "Forerunner 645 Music" device from Windows explorer, and paste it under primary\garmin\apps
Oh, if you want to copy a .set from the sim to a watch for testing, on the 645m, the name is case sensitive, so if the prg is "Abc.prg", the .set from the sim may be ABC.SET, so you want to fix the case.
With windows, it understands MTP devices (I've used Windows Media Player to put music on the watch, for example), but macs don't understand MTP, which is why another app is needed to access the 645M.
For a Mac, it's looking like I might have to pay for something like SyncMate to get MTP transfer going (not seeing a native/free mechanism that would expose file-like access).
