BDc is Bottom dead center. if you remove the plug and look in you will see when it is down and you should be able to feel the point when the piston is at the bottom.

Put the afterrun oil in through the glowplug hole and turn the crank over by hand a few times to get the oil down through the engine. Just remove the manifold, pipe and airfilter from the engine and bung up the holes with some tissue paper to keep the dirt and dust out.