Just a 2-prong (European style) plug is all you need. I've never had any problem getting power, but when my mom flew ATL-JNB and JNB-JFK in February, the outlets didn't work either leg. (The plugs were correct, but there was no power to any outlet on either aircraft.)

Enjoy your trip!

I hope it's good, since I'll be making my first IAD-JNB flight in 6 weeks.