Ah, there is no "Add". The Add functionality was there in a very old version of NGUI as the original way of defining sprites -- before the Atlas Maker was ever created.
Texture packer can save your atlas definition as TXT, which you can then import as an atlas. You say you didn't use it, so your other option is to use Unity's atlas-creating functionality. You can create a sprite sheet from a single image using Unity 4.5 2D sprites. You can then draw them using NGUI by using UI2Dsprites as I mentioned in the previous post.