Get a digital multimeter (DMM) and check for continuity between the ground and at post and then again from the post to the alt case. If there is good continuity than that posts used for a ground so you might be able to ground it out in order to close the circuit.
Where did you get the alternator at? If it doesn't work in your car because something is missing or added than it's probably not the correct alternator. I'd check that first.
Good luck and let me know what happened.