Which one is ground?
From your readings, the middle terminal is isolated from the others, and I would surmise that it's either earth ground (if your AC adapter has a ground terminal on it) or simply disconnected, so it can be ignored.  For DC power all you need are two terminals, one each positive and negative.

What's what most people call "ground" is determined in context of the circuit you're powering, and it's usually negative--here, the right terminal.