Im having issues with my 3dr radio. Every time i plug it in and try to connect or use the config utility i get “invalid comport”. I have tried 3 windows machines i got no luck. Baud rate is set to 57600. And ive i stalled and uninstalled the drivers and apm 2.0 numerous times. The radio works on my android phone and even connects. It just doesnt work on any windows machines. I have a really old mac im trying to get to work to see if i can configure it and get to work there. But any help would be greatly appreciated! This problem is making me pull my hair out its getting pretty annoying.
Thanks! 
Are you using the mission planner 2 radio config or the 3DR radio config?
Should be as easy as set com port (in windows check you device manager in control panels) that will tell you the com port it’s on and then it’s connect as long as both the radios are configured with the same parameters on both then you should see the green light go SOLID and the red lights start to flicker with data transmission. cant say I can remember the older mac serial setups but should be just the same 8-N-1 for com port settings and then go into radio config and connect.
Hope this help