If you do not have a MyAccount, you can still edit the settings on your domain. As long as you have the registry key for the domain, you are able to log in and change nameservers, edit contact details, change the owner (for some domain types) and lock/unlock the domain.
To access the domain management screen, go to the main page of the Melbourne IT website and click the MyAccount icon at the top of the screen. One of the options available is "Log into domain maintenance". If you select this option, you will be prompted to enter the domain name and registry key, after which you will be taken to the management screen.