I'd like to be able to include the map legend in the cascade template but I do not see an option to do so.
Dan's right, Cascade doesn't include the ability to show a legend at this time, and the resources he pointed to are helpful to show you how you can explain your maps without legends in a Cascade or any story map.
It sounds like you are also trying to give your audience some control over the visible layers in the map. We typically don't recommend including standard GIS-style layer controls in story maps. Story map builders are designed to help you, the author of the story, lead your readers through your maps in a deliberate and directed way. In Cascade, for example, you can use the same web map in several views of an immersive section and change the layers shown (or move the map to another area or even show pop-ups) in each view, so all your readers need to do is scroll and the app will adjust what is shown in the map to match your story. Here's an explanation of how to use this technique: http://arcg.is/29ig7Db
Map Journal also has a similar capability called Main Stage Actions, which you can read more about here: Using Story Map Journal Main Stage Actions | ArcGIS Blog
There are some situations where a layer control may be useful in a story map:
In these cases you can embed a configurable app like the Basic Viewer or Web AppBuilder in your story map to include a layer control, but even in these situations it is still often better to link out to one of these exploratory apps at the end of your story (so they open in a new browser tab) rather than embed them inside the story.
Hope this is helpful!Owen
apparently not yet... but this blog provides some insight Mapping in Cascade...Without Legends
Retrieving data ...