You need to locate the file postgresql.conf with the following command:

find / -name "postgresql.conf"

in this case the file is located in /etc/postgresql/9.2/main/postgresql.conf but this will vary depending on your installation method

Edit the file postgresql.conf, change and uncomment line below:

listen_addresses = '*'

Next you need to add the IP of the machine you want to allow access so locate the file pg_hba.conf:

find / -name "pg_hba.conf"

edit the file and add the ip of the machine you want to be allowed to connect:

host    all             all             1.2.3.4/32            trust

Restart postgres and you will now have access!