What you plug in your USB cable to your PC and X350 quadcopter, do you hear any USB connect sounds? If not then there might be an issue with the cable not being plugged in properly or the cable itself is damaged
However the second video in this post shows a great overview on installing the software and drivers for your X350 flight controller.
The great thing with using mission planner is that it also installed the drivers for the board which I think is possibly the issue you are experiencing.
Hope that helps