I recently installed Linux Mint (replacing Ubuntu) and have now installed postgres using, step by step:
sudo apt-get update
sudo apt-get install postgresql-9.3
sudo apt-get install postgresql-contrib-9.3
sudo apt-get install postgresql-doc-9.3
sudo apt-get install pgadmin3

I can start and stop the server manually and postgres works fine.
However, the postgres server does not start up at boot. When previously using Ubuntu the server auto-started, but not with Mint.

I am not familiar with the technicalities of run levels, however I have found:
/etc/rc3.d/S19postgresql -> ../init.d/postgresql
/etc/rc5.d/S19postgresql -> ../init.d/postgresql
/etc/rc6.d/K21postgresql -> ../init.d/postgresql
/etc/rc0.d/K21postgresql -> ../init.d/postgresql
and have confirmed the existence of:
-rwxr-xr-x 1 root root 1407 May 11 2013 init.d/postgresql

I have also added:
and set it to "auto"

Still no joy. Any clues?

Linux Mint 17 Qiana
postgresql 9.3