The sticker on the bottom of the machine tells what power adapter to use:
Pro Pow = 5V (3A)
Pro Pow 2 = 5V (3A)
Pro Pow E = 9V (1.2A)
Pro Pow 3 = 10V (1A) or 11V (0.7A)
Megadrive PSUs are either 9V or 10V but you have to be sure the plug has the correct polarity for the Neo (tip negative, outside positive).