can't access postgres file pg_hba.conf

First note down the location of pg_hba.conf on your system. If you don't know it but you can connect, connect to Pg with PgAdmin-III or psql and run:

SHOW hba_file;

to get the location of pg_hba.conf. It's location is pretty standard; on Ubuntu it'll be in /etc/postgresql/[major.minor]/main/pg_hba.conf eg /etc/postgresql/9.1/main/pg_hba.conf. On most other distros it'll be under /var/lib/pgsql/ or /var/lib/postgresql, either directly or in versioned directory.

To edit this file you must specify the full path or change directory to its location first. Say:

sudo vi /etc/postgresql/9.1/main/pg_hba.conf

If you prefer a friendlier text editor:

sudo nano /etc/postgresql/9.1/main/pg_hba.conf

or replace "nano" with your preferred editor, like gedit.

Once you save your changes, remember to reload the PostgreSQL service to have them take effect. On Ubuntu you want:

pg_ctlcluster 9.1 main reload