Under your mod's dropdown, hit the + symbol next to 'Skill Data' and select the type of skill you want to add. Anything you add there will be added to the game (or modify something that already exists) if your mod is active.
You'll also see a dropdown for your mod with nothing in it. Here, you can see (but not directly modify) all of Divinity: Original Sin 2's items and skills. In the upper-right hand side of the menu bar, you'll see a bunch of different icons for opening editors.
