The leads on your voltmeter are black and red. Red is for the hot and black is for ground/neutral. It shouldn't matter which lead you use to test with when working with A/C current. However, on a DC circuit, it will read negative if you don't use the correct leads for the correct wire. Did you have your meter set to DC? Most likely, you bumped into something with your test leads to cause the short.
Anyway, if the lamp post wires lead out and have no voltage, they must have been previously disconnected. Trace them back into the box to verify and/or perform a proper termination on them inside the panel.