First off, to dropdb, you should su to the owner of the database (su nsadmin), if still fails (which is VERY unlikely), you should be able to do it with the user "postgres". If don't know / not sure about the passwords for these users, try su to root and then su to these users, that way you only need the root password. Try "dropdb openacs" again when you are clear what user you are running the command in (the database name for the RPM installation is "openacs")
What do you mean by creating a new user in your 2nd question? If you mean creating database user, there is no such need. The RPM installation default checked and created two users for you: "postgres" and "nsadmin". "postgres" is the super user for the postgreSQL database while "nsadmin" is the owner of the "openacs" database, which is where all the openacs data being stored. If you want to list the database tables, "su nsadmin" and "psql openacs", that will give you the psql prompt. You can then "d" and display a list of database tables in the Openacs installation.
If you mean creating new ACS users... just go to your site's URL and the appropriate port (most likely 8080 if you use everything default for the RPM)
Hope this help.
