1. On the Radio, choose "Home > Settings > Advanced > Remote Login > Enable SSH". You should see a message telling you the root password. If you don't see the message, restart your Radio and try again.
2. From your PC, SSH into the IP address of the Radio with root as the user. (Windows users can use PuTTY to SSH.) For example:
$> ssh 192.168.0.2 -l root
3. Enter the password (the default is "1234").
You can change the password using passwd after login. If you forget the password, you need to reset the Radio to factory settings.
Thanks to the various users in this forum thread.