Andrew left a few things out.

First thing I'd do is see if the path from the alternator/ wiring/ bulkhead/ ammeter/ battery is good. So as andrew said

WITH engine simulating "low cruise" jumper from the big alternator stud (output) directly to the battery, see if the voltage goes up. Double check by checking the voltage at the battery, and at the stud. They should be close, within 1/2 volt or so

If this is good, see if the alternator will charge. Here's how these work:

Both the regulator ign terminal and ONE terminal of the alternator field "get" switched ign from the key. On a stocker this is the blue wire, so with the ign in run or the engine running, make sure the ign terminal of the regulator is getting battery voltage, and identify which field terminal is doing so--if it is. If so, hook that one up, and disconnect (normally green) wire from the opposite field.

Now with the engine running low-to med. cruise, GROUND the exposed terminal of the alternator field. This should give you battery (12V) from the blue--through the field--to ground. THIS SHOULD CAUSE the alternator to go to "full output" You should see a dramatic rise in voltage at the alternator output stud AND AGAIN this voltage should be close to the battery voltage, say, within 1/2 or as much as one volt difference FOR NOW

If you get this far, and it charges, it's starting to sound like the regulator.

If it does NOT, shut everything off, take your meter, and see if you have continuity through the two field terminals. You MAY just have stuck/ worn brushes, an easy fix. If you DO have continuity, and you did not get any output, verify that the field draws current by jumpering power to it--battery to one terminal, ground to the other. You should see a small spark. Double check, and if no output, you probably have diode/ stator troubles==new alternator

Remove the regulator, scrape the ground clean, hook it up and make sure the connections are tight and double check it. If you are sure it's grounded, if you are sure the (normally green) wire from the regulator to the one field terminal is "continuous" and if you are sure that you are getting "ign run" (battery) voltage to the other regulator terminal, it's time to try a new one.