For future reference, here are the steps to add a SSH key to a CoreOS guest machine's authorized_keys:
wget https://raw.github.com/coreos/scripts/master/contrib/create-basic-configdrive
chmod +x create-basic-configdrive
./create-basic-configdrive -H HOSTNAME -S ~/.ssh/YOURKEY.pub
5. boot up CoreOS VM then press Enter a few times (as necessary) until the machine's IP address is displayed
NOTE: if IP address is not displayed then adjust the machine's network settings as required
ssh core@IP