To add a device to your amazon account you need to install the Amazon app and then click on the app to launch the amazon app.
Be sure your device is connected to wifi or another data connection and then sign into your amazon account on your device.
There should then be an option to add or register that device with your amazon account once in the app.
The actual process usually varies between devices so you'll just have to follow the instructions in the app once you're in your amazon app.
You may also have to sign in to your amazon account and add the device through your computer as well.