When you are an Applicant (sometimes referred to as a Prospective student), Admissions will assign you username consisting of your USD ID number (such as 008123456) and a PIN number. This will allow you limited access to the MySanDiego portal for the purpose of checking your application and financial aid information during the application process. However, you will not be able to register for housing or create an e-mail account until your deposit has been received by the university.
Once you have been accepted to the University and your deposit has been processed, you are allowed to create your full MySanDiego account. To do this, you can visit http://my.sandiego.edu and click on the Create Account link, which you will find directly underneath the log in fields. When creating your account, follow the instructions on the page an provide the requested information. Once completed, you will have a full account (such as email@example.com). Once your account has been created, it takes between 24-48 hours before you will be able to access all of USD's systems. During the delay, you may not be able to log onto ANY USD systems, so make sure you don't create the account the day you need it.
Once the 24-48 hour period has passed, you will be able to access the Housing portal (aka StarRez). If there is a housing application available, you will be able to fill it out.
To recap, if you already have a full email address (i.e. mbaek-12) you should be able to log in to the Housing portal. If you have an ID number (i.e. 008208499) for a username, then you can't.