Africa is a continent with at least 54 countries and over 1 billion people living there. There is not one capital for the entire continent. There is an organisation though that represents most of the African nations. It is the African Union.
The African Union is a very influential organisation. Its headquarters are in Addis Ababa. The capital of Ethiopia would be the closest thing to a capital of Africa. This is assuming that the location of the headquarters of an organisation that does represent most of a continents nations is a good indicator for a continents capital.
If one does go by that definition Addis Ababa is the capital of Africa.