AN ELECTRIC MACHINE IMPORTED FROM KOREA HAS A GREEN, A BLACK AND A WHITE WIRE
GOING IN TO THE PLUG. THE GREEN WIRE IS EARTH. WHICH IS POSITIVE OR
NEGATIVE OF THE WHITE AND BLACK WIRES?

Firstly, the machine is most likely to be powered from alternating
current (ac) not DC. Hence there will be no plus or minus. However
there will be an active and a neutral. I understand that in some parts
of the U.S. The neutral is white and the active is black. (I'm from a
240 volt country)

So, best guess would be that it is for 110 volts.

Better advice would be to leave this alone until you understand more
about electricity.