"Show something to somebody" is mutat valamit valakinek. So if you want to include yourself or someone else as the recipient of the action, you'll have to put them in the dative (an indirect object), not the accusative. Thus:
"Engem" is me in the accusative. So then I am the object being shown. As opposed to "nekem", when the object is being shown to me.
But the sentence does not say anything about the person that János is showing something to.