First, the method for adding users varies from one Unix to the next - I know three off the top of my head and they are all wrong for OS X. Remember, OS X is based on Mach and FreeBSD, not Linux.
Second, admin privileges is not the same as root access. Just because you have admin rights doesn't mean you have the powers of "super user". ;-)
If you really want to fiddle with OS X from the command line, I would suggest O'Reilly's book "OS X Panther for Unix Geeks" - it will explain how OS X differs from other Unixes.