su postgres
Then create a new user:
createuser jim
Enter user's postgres ID or RETURN to use unix user ID: 500 ->
Is user "jim" allowed to create databases (y/n) y
Is user "jim" a superuser? (y/n) y
createuser: jim was successfully added
You can use either the UNIX user ID or the postgres ID to identify users. See the postgreSQL documentation in the "admin" section for a fuller account of users and groups.
The reason for using the command line to create the first user (other than postgres) is that postgres may not have X-Window permission. You can also create users using PgAccess if you are created as a superuser as shown in the example above.