You need to enter the password of the *database* user, not the OS user. The "root password" is not applicable. Do you know the password of the database user `postgres`?

Most systems are set up allowing [peer authentication][1] for *local* connections, meaning a system user can log into the database with a database user of the same name without password. `localhost` is *not* a local connection. Its privileges are determined by `host` entries in your [`pg_hba.conf` file][2].

Since you seem to be running Postgres and pgAdmin on the same server, I suggest you try a *local* connection *without password*. [Quoting the pgAdmin manual:][3]

> On Unix based systems, the address field may be **left blank to use the
> default PostgreSQL Unix Domain Socket on the local machine**, or be set
> to an alternate path containing a PostgreSQL socket.

Bold emphasis mine.  
Of course, you have to run pgAdmin as the system user `postgres` for this to work.

[This related answer on SO][4] has a lot more details and links.


  [1]: http://www.postgresql.org/docs/current/interactive/auth-methods.html#AUTH-PEER
  [2]: http://www.postgresql.org/docs/current/interactive/auth-pg-hba-conf.html
  [3]: http://pgadmin.org/docs/1.18/connect.html
  [4]: http://stackoverflow.com/questions/15359348/run-batch-file-with-psql-command-without-password/15593100#15593100