I installed APMPlanner 2.0.20 on Windows X (x64) and had a similar problem. You need to convince your computer to assign a COM port to a USB location.
Plug in the USB device (drone and/or 915MHz transmitter) & reboot. Windows should discover and install the device(s).
Check under PORTS in Windows Computer Management. The IRIS+ will be added as a USB serial device and will be assigned a COM port number (i.e. COM5). The radio transmitter will be listed as USB serial port and will be assigned a COM port number (i.e. COM4).
There may be operational issues or missing driver components … select each added device and use TROUBLESHOOTING. If issues were found, they should be automatically fixed.
Go to the Control Panel and view installed devices and printers. You should see something like FT231X USB UART. This is the 3DR 915 MHz radio transmitter. If you connected the IRIS+ directly via USB, you will notice a serial device (this is the onboard Pixhawk).
When this work is completed, Windows X will allow you to communicate with APM Planner by direct connection or by 915 MHz by selecting the assigned COM port from the drop-down list.
I hope this helps you and others with the problem.