tcp ports 512, 513, and 514 are known as "r" services, and have been misconfigured to allow remote access from any host (a standard ".rhosts + +" situation). to take advantage of this, make sure the "rsh-client" client is installed (on ubuntu), and run the following command as your local root user. if you are prompted for an ssh key, this means the rsh-client tools have not been installed and ubuntu is defaulting to using ssh
the root account on metasploitable is msfadmin so well log on using that account. the password is also "msfadmin