I can't find anywhere in the D365Mktng App to to get to the Marketing Configuration entity.  The documentation says this:

"Use marketing configurations to enable Litmus previews

Use marketing configurations to set a few overall settings that affect the way the app works. Currently, these only include the ability to enable Litmus for email previews.

Go to Settings > Advanced Settings > Marketing > Marketing Configuration to open a list of available marketing configurations. You can store as many configurations as you want, but only the one marked as Default is active. Choose an existing configuration to edit or delete it; choose + New from the command bar to create a new one."

But I can't see this in the settings.  One does not appear to be able to enable Litmus Previews from the App without this.