The only thing that I know to fit in the lower slot is a IC2 battery which does not last long at the 6eu/t the engine uses. As for using the engine just wire it up to a batbox or similar low volt energy source such as a generator than apply a redstone signal to it.
Please be aware that electric engines are horribly inefficient compared to other engines unless you have a lot of EU to spare.
The slots in the engine are for IC2 handheld power sources (energy crystal/lapotron ect) and forestry circuit boards (which buff the power output or Eu:MJ conversion)