You could find the IP address using
nmap's ping scan), but could also use the default
/etc/hostname alarmpi in place of the IP address if DNS is working in your network.
If your DNS is not working, then do
nmap -T5 -sP 192.168.0.0-255 replacing the network id part of the address accordingly.
to get a list of IP addresses using the IP address of the currently active network device: Results will be IPs that share the same network (subnet).
After that, read the Raspberry Pi Installation Guide at the bottom.
It is only tricky in that it fails to mention that you cannot login as root on a headless Pi.
Root login is disabled by default so you need to use the default Arch Linux ARM user/password, see Where does the word "alarm" come from?:
Existing non-root username: alarm
Associated Password: alarm
Therefore, in your example you'd want to type
ssh firstname.lastname@example.org or just
ssh xxx.xxx.xxx.xxx. Fill in the
xs with the IP address of your Raspberry Pi.
Once you are logged in, you can switch to the root user:
login or as an su subprocess
su -l root, in both cases the password is root.