If using a Macintosh computer, additional software is required in order to transfer files.
Connect the device to a computer using the supplied USB cable.
If necessary, touch and hold the Status bar (area at the top of the phone screen with the time, signal strength, etc.) then drag to the bottom.
The image below is merely an example.
Tap the USB icon then select Transfer files (MTP).
Selected when an indicator is present.
Launch File Explorer or Windows Explorer.
Using the computer keyboard, press Windows+E ( +E) keys.
Alternatively, right-click Start then click Open File Explorer or Open Windows Explorer.
Navigate: Nexus 6 > Internal Storage > Music.
Copy the desired music and other audio files located on the computer into the folder.
For any accompanying artwork in JPEG format, rename the art file albumart.jpg and copy into the same folder as the music files.
To further organize files, create a subfolder in the newly created music folder and copy desired music files into it.