Andy from Ultimarc will probably chime in here with more detailed advice, but I ran into this exact same problem when updating my Aimtrak firmware too.
Updating the firmware on your Aimtraks is a 'two step' process. You've completed step one, but you still need to complete step two.
Step one erases something in the onboard memory of the device and prepares it to accept the new update. Your Aimtrak will now identify itself as 'UHID Firmware Update' until you run the SECOND part of the update.
I believe the program you want to run now is called UUpload.exe, and it's located in the Aimtrak folder you created when you downloaded and installed the Aimtrak software. Most likely it's in C:\Program Files\AimTrak or something close.
Run the program with the device connected, select the firmware file (in the same folder as the program, I believe), and you should be good to go.