Jesus is not on earth. How can he be the federal head when He is not HERE?

This smacks of the Manichean heresy of dualism: that the physical body is bad and the spiritual is all that really counts.

Where is this in scripture?

It is taught in principle in the OT and the NT follows suit. God was the King of the Jewish nation. But while God was in Heaven, it was the high priest who was the covenantal head over the nation. As I mentioned before, the sin of the high priest as federal head over the Jewish theocracy is what doomed the Jews to near extinction in AD 70 when the judgment of God fell upon them (Matt. 23 - 24).

Brother Ed