First, decide which endpoint you will be using. You can see list here: BackupSheep Endpoints
You will need the endpoint IP address to configure access. Replace ENDPOINT_IP variable with IP of your selected endpoint.
Login to your server using SSH.
$ find \ -name "postgresql.conf"
Open postgresql.conf file and replace line
listen_addresses = 'localhost'
listen_addresses = '*'
open pg_hba.conf and add following entry at the very end.
host all all ENDPOINT_IP/32 md5