[chirp_users] Downloading problems

Hi I am having problems downloading from radio .I have arcshell5 and an rt cable .They said it would work with a Mac. I have Sierra operating system.I get an error message saying an error has occurred no response from radio. Or when I chose another choice I get an error that includes the name of one of our computer.an error has occurred could not open port/dev/cu.velosia2/-serial port-4{ermo/6}resource busy:/dev/cuvelosia 2-serial port-4. Thank you for your help.