My Layers are not Showing in Preview?

I am just starting with Storyline and have been going through the tutorials fairly systematically.  Though several tutorials talk about layers, and I believe all include discussion about getting to a layer using a trigger, I do not recall anytime where it is explicitly stated that using a trigger is the ONLY way to get to another layer.  I can't think of any other application where this is how layers work - Photoshop and Flash are just a very short list of candidates.

If I am right, then my suggestion to the folks at Articulate would be to add a note somewhere in the tutorials that layers can only be accessed via a trigger.  For instance, in the tutorial "Adding and Editing Slide Layers", change this sentence: "Once you've added layers to a slide, you can create triggers that tell Storyline when you want a specific layer to be revealed."  As I understand things now, it is not 'you can create triggers', it is 'you must create triggers'.

Sandra's reference to Flash and Photoshop layers were my trigger to realizing why my layers were not showing up in Preview/Publish despite any number of permutations and convolutions over the past several hours.   Thank you Sandra.