Why can't I get Android File Transfer to work on my new Samsung Galaxy Ace 4 via USB?

[AC Question]

Why can't I get Android File Transfer to work on my new Samsung Galaxy Ace 4 via USB

Samsung Galaxy Ace 4
Android 4.4
Apple iMac/Yosemite 10.10

I have tried everything- uninstalled Kies, reinstalled ATF for Mac, checked the USB cable and its socket (both work), the phone tells me that it "Is connected as a media device", tethered/untethered USB..... yet whenever I launched ATF I get the error message "Could not connect to device. Try reconnecting or restarting your device"

Still doesn't work when I do that.

Frustrating!

Pls help as I need to transfer some large files.