On linux platforms, you'll need to install UDEV rules in order to allow non-root users to communicate with your KeepKey:
$ curl -OL https://raw.githubusercontent.com/keepkey/udev-rules/master/51-usb-keepkey.rules
$ sudo mv 51-usb-keepkey.rules /usr/lib/udev/rules.d
$ sudo udevadm control --reload-rules
Then unplug & replug your device. You do not need to restart your machine.