If a layer isn't appearing in your Map Layers panel, it may be disabled in your settings. Click the Manage Layers button at the bottom of the Map Layers panel to open the Manage Layers dialog, where you can enable any hidden layers.
How do I customize which layers appear in my legend?
Click the Add or Edit Layers button at the bottom of the Map Layers panel to open Manage Available Layers dialog. From there, you can enable or disable any layer. Disabled layers won't clutter your panel but remain available to re-enable anytime.