I tried searching for this answer and reading through FAQ's, no luck.
I just downloaded/installed/imported nGui 3.6.0 (never had a previous version before). I'm watching Tutorial 2 (
https://www.youtube.com/watch?v=aWZaq1eLqyE) and on his Inspector under UISprite script he has a "Sprite" button he uses to set the sprite to something. I don't have this button. I tried a new scene and re-added a sprite and nothing. Also I don't know if it's normal but when I click on "Edit" next to the Atlas button, Wooden Atlas I get an error NullReferenceException: Object reference not set to an instance of an object
UISpriteInspector.ShouldDrawProperties () (at Assets/NGUI/Scripts/Editor/UISpriteInspector.cs:63)
Not sure if they are related.