Hi Rob,
To login as root user you need to follow these steps,
1) SSH to the instance using "ec2-user" and key associate with the instance.
2) Once you are in the instance as "ec2-user" , "sudo su" to convert to root user.
3) Now you are running as root user and for changing to hdbadm user run "su - hdbadm" .
Thanks,
Priyank