You don't add them in here. You add them into the Universe in MyDMX. Then they can populate into the 3D Visualizer.
I'm having issues doubling up on certain fixtures, but I may have to delete them and re-add them in my Universe. I have 32 Par38's, but I use 16 DMX channels(2/channel) on my universe. I operate them in banks of 4 lights, and aim them in single zones of 4 lights each(red, green, blue, yellow). But since I'm phasing these out, I don't stress over them much.