Use the --datadir option
On a Mac, it is hard to convince the app to take a command like argument. At least, I could not get it to work.
I posted an alternative solution here:
https://bitcointalk.org/index.php?topic=1680135.msg17038225#msg17038225NOTE: This will move Bitcoin Core's data. With a recent Armory version, it is only Bitcoin Core that has large amounts of data.