How do I do a simple division in Numbers? when I hit the / key in numbers it comes out as a +

Division ( "/" ) doesn't work since up-grade to El Capitan

If you look more closely, it is a division symbol ÷, not a plus sign. I suppose Apple thought it looked cleaner than the "/" that other apps use and never considered that maybe it looks way too much like a plus sign and will result in confusion.

You have to look very closely. That symbol is actually the classic division symbol. you are not doing anything wrong. Check the match with simple numbers to convince yourself