A lot has changed since the original 2017 post. You no longer add devices using the app.
Instead, hold the Set button on the HomeKit hub for 5 seconds and the light on the front will start flashing green. Next, hold Set on the device being added. It’ll beep, and both will stop flashing. In a couple of minutes the new device will show up in the HoneKit app where you can rename it and add it to the appropriate room, group and scenes.
To remove a device, hold the Set button on the Hub for 5 seconds until it flashes green. Let go. Hold again for 5 seconds until it starts flashing red. Hold the module’s Set button for 5 seconds, both will beep, and the module will be removed from HomeKit.