As stated above, voltage should be right about 14 with the engine running.
Depending on your pulley setup it may take a few RPM to get the voltage up. Try revving the engine to 2k and see if there's still no change.
If it's still 12ish at 2k then get the alternator tested. While testing make note if the guy hooks up the Field wire to excite the alternator. Once you've ruled out the new alternator being DOA you can proceed with wiring.