It sounds like either a driver issue or maybe needing to run 'sudo' (if you're on a Mac, or Linux computer). You can check and get a list of dfu'able devices by typing:
#OR if permissions are needed
sudo dfu-util -l
# if you see a device showing up as "1d50:607f"
sudo dfu-util -d 1d50:607f -a 0 -s 0x08005000:leave -D choosatron-core-v100.bin
Can you give that a try and let me know if that helps?
If you're on Windows, try opening the Command Prompt as an Administrator.