Empty, uneditable pg_hba.conf file

Question

I am trying to connect my Sinatra app to PostgreSQL database with this tutorial: http://samuelstern.wordpress.com/2012/11/28/making-a-simple-database-driven-website-with-sinatra-and-heroku/. Everything goes fine until I try to execute:

rake:db migrate

then I get this error:

rake aborted!
PG::ConnectionBad: fe_sendauth: no password supplied

It seems like I should change my permissions in pg_hba.conf file, but, opening it, I see nothing. And, if I try to write something into it, it says it's "readonly".

Am I on the right way of getting rid of the error and where to go next?

edit: actually, if I try to open it from another text editor, it says I have no permission to.

Answer 1

So pg_hba.conf was empty because I was trying to open it with SublimeText and sublime didn't tell me that I was not permitted to read it, it just showed me an empty file.
And to edit pg_hba.conf I needed to open it through terminal

/etc/postgresql/9.3/main$ sudo subl pg_hba.conf

with sudo.