PostgreSQL 7.2.1 on OS X -- psql: FATAL 1: user "postgres" does not exist

I have installed the PostgreSQL package from the Apple site (7.2.1)
and ran the install. Everything seemed to go well. The files and
directory structures are there, but I cannot get anything to work.

I keep getting a FATAL 1 error with any user I try to put in. I cannot
use createuser either... even my root account cannot overcome the
error.

Any ideas?

I am definitely a novice and I am trying to build a patient tracking
system for the ER that I work in.

Thanks in advance.