In order to set up the Lightstream Twitch Alerts, you will have to navigate to the "Produce" tab by clicking the logo on your right side of the screen. Scroll down until you see Twitch Alert section and turn it on (in order to use the Alert, you will need to link your Twitch account and set it as a streaming destination).
If you want to see how the alert looks like on stream, you can click the "Eye" Icon and click it. The test alert will be broadcast to the studio canvas.