You don't need a voltage converter (there is no such thing as a "wattage converter") if you're using an Apple laptop.
If you look closely at your power brick, it says somewhere "100-240V" or so, meaning it will run on anything between the 100/110V you get in Japan and the US, and the 230V you get most everywhere else.
All you need is a physical adapter to be able to plug the thing in - which you can get at Radio Shack, probably.
Alternately, the little "corner piece" of your power supply comes off, and the socket in there will fit any standard electric shaver cable, so you could just try to get one of those in India.