Hi Michael,
Thank you for contacting Sony Alpha Email Response Team.
I understand that you can’t update the firmware of your camera using your 10.10.3 Mac computer.
I am sorry for the inconvenience. Unfortunately, our firmware updater is not yet compatible with Mac OS X 10.10.3. Our software engineers are still update our software in order for it to be compatible with the latest Mac OS version. I suggest to use a different computer like lower version of Mac or a Windows computer to update the firmware of your camera. You may visit our eSupport.Sony.com website for any news and updates regarding our firmware update.