So you need to manually install the firmware in the Zigbee updater because it doesn't automatically grab the latest firmware anymore. There is a Mac/PC directory where you install the firmware and the updater reads it.
Then, via USB you download the updates to the ZIM. ALWAYS install RADIO firmware first. Then, assuming the ZIM is connected to a thermostat, it will automatically update the thermostat. I think RC2000 radio firmware is 15.08 and the main RC2000 is 15.07. So you need it all running and connected before it can be updated.
Were you successful updating the RC-2000? Would you be able to share the firmware and updater for that process? I have PC Access, but just switched from an old RC-80 to an RC-2000 after upgrading my HVAC system.