When it comes to adding layers you don't have to do a thing... well not technically. Layers are automatically added each time you add an element to your scene.
Did you add a poll? A layer with your poll will automatically populate in the layer list on your right-hand side. You've added some audio? That has a layer too, where you will also be able to play and pause. Video feeds, Chat, Banners ... everything you add to build your scenes will automatically be added as a layer.
You don't have to do anything or worry about adding layers on your own ever.
We recommend keeping your Layers menu open while broadcasting. In case you lose it, just click the Layers & Scenes icon right under the Start Streaming button.