Connecting a Galaxy S3 to my MacBook Pro 10.10.5
I have faced the exact same problem when I first started android development. My One Plus One would simply not show up in Android File Transfer. I would highly recommend that you re-install Android File Transfer.


Install Android File Transfer app.
Reboot your Macbook Pro after installation
Plug in the usb cable into your mac and start android file transfer.


Let me know if this helped or not!