You need to set Core's data directory using the -datadir parameter. Looking at this troubleshooting guide it looks like you'll need to start Core manually using the -datadir parameter before launching Armory:
https://www.bitcoinarmory.com/troubleshooting/#how-can-i-change-the-blockchain-database-storage-location-for-armory-and-bitcoin-coreSince MacOs X has a different concept of shortcuts than windows, you'll either have to launch Bitcoin Core from the command line like so:
/Applications/Path/To/Bitcoin -datadir=/Path/To/ExternalHD/
...or create a script, like so:
https://superuser.com/questions/16750/how-can-i-run-an-application-with-command-line-arguments-in-mac-osAdditionally you'll need to start Armory using the --satoshi-datadir parameter, pointing at the same /Path/To/ExternalHD/ (either via command line or script, similar to Bitcoin Core).
What exactly do you mean by armoury? The wallet,
https://www.bitcoinarmory.com/ ? You should be able to just download that for mac, and it should work fine.
Ofcourse you can also use bitcoincore, but that seems unnecessary to me, if you already use armory..
What am i missing, what isn't working?
You can't run Armory without Bitcoin Core. (unless this has changed since I last used Armory, which is admittedly 1-2 years ago)