Having a login name for the ISS PC service does not necessarily mean that you have access to the Unix service.
Staff and students who are not preregistered for ISS services need to visit ISS Reception to register. You will need your University smartcard.
Your login name has a password, without which it cannot be used. This password gives you access to all ISS Unix systems. The first time you use a newly-issued login name you are required, for security reasons, to change the password immediately, and you are guided through the process.
If you also have a ISS PC login name, note that although your login name is the same, the two services are entirely independent: changing your password on one service does not change it on the other.