FYI anyone running into this error ~recently:
"Could not connect to the everdrive. Please make sure the USB cable is plugged in and the NES is powered on"
Be sure to check OS updates, I was hitting this on my setup that worked few months back - same cable/everdrive/NES. Tested OK with same setup but another (recently-used) laptop & synthNES loaded fine. Caught my synthNES laptop up on updates & it was able to 'see' the NES again.