You cannot use the postgres account without some tweaks, Sorry I don't remember where I found this info - somewhere in the pgadmin install docs.
If it does say that anywhere, I'd love to know where so I can remove it.
However this works:
Create a new superuser with a password, add connectivity to the pg_hba.conf file if needed and pgadmin should work fine
The postgres account should be quite usable for this. You might need to edit pg_hba.conf as well of course.
Regards,. Dave