Can't Edit Tile Map Xcode 8 GM

I can't for the life of me figure out how to edit my tile map in Xcode 8 GM. Double-clicking does nothing, and in the Editor dropdown "Edit Tile Map" is greyed-out.

All I've done is drag-and-drop the tile map onto the scene, and even creating a new project and starting over has not helped. Every video/tutorial I've seen has just shown adding it and double-clicking to edit... Is there something really simple I'm missing?

You need to specify a Tileset. See the drop down in your image, it says Invalid Value