In order for Jenkins to send notifications to your Android device, you need to register the device with Jenkins.
If you're using the BuildWatch app, you can click the "Show QR code" button here, then scan it using the "Auto-setup" feature in BuildWatch.
Clicking "Enter token manually" shows you the currently-configured device token. If you configure a new token using the app, then you will need to refresh this page before you see the updated value.
A video walkthrough of the setup process can be found here: youtube.com/watch?v=mt0ctjBTiZw