Enable SSH access for Synology DSM

This assumes you already have an SSH client installed and set up.

From Control Panel -> Terminal & SNMP, select "Enable SSH service"

Next, enable "user home service" at Control Panel -> User -> Advanced tab, so that users can actually SSH login to somewhere useful:

Next, enable permissions for SSH-capable users to their "homes" folder:

You can now log in via terminal like:

ssh username@ip-address

Be sure that your user has login privileges. If they don't, SSH in as root, and set them up:

ssh root@ip-address
//...Logging on to the NAS...
sudo vi /etc/passwd

Change the end of the listed user's line from /sbin/nologin to /bin/ash to permit login.

Interesting read regarding security:
http://www.eldemonionegro.com/blog/archivos/2012/08/19/how-to-securely-activate-ssh-into-your-synology-diskstation-with-ssh-keys-and-no-root-login