I was momentarily lost when adding a ground plane to one of my PC boards at a friend’s suggestion… for the help of others who may wind up in the same pickle, here’s the steps.
When in board setup, select the number of desired layers (Hint: When using an external autorouter, this step may be best done AFTER letting the router have a go with laying down traces), and assign the “power” attribute to those layers as needed.
Back in the main screen, make one of these new layers active. Select the zone fill, and you should be prompted for the ‘net’ to assign to that layer. From there you can adjust the zone shape to your needs, but remember to refresh the fills (B) whenever routing vias through the layers.