There is no configuration file present in /etc/davical/config.php (or in www.otenet.gr-conf.php) so your installation is not fully set up.
Well, you're seeing this! At least you have DAViCal installed :-) You also have Apache and PHP working and so really you are well on the road to success!
You could try and click here and see if that enlightens you at all. Odds are it's a fairly broken link, but it might work sooner or later so keep downloading new versions and trying again. Or make some guesses. Or bug Andrew :-)
The configuration file should look something like this:
<?php // $c->domain_name = 'davical.example.com'; // $c->sysabbr = 'davical'; // $c->system_name = 'DAViCal CalDAV Server'; $c->admin_email = 'email@example.com'; $c->pg_connect = 'dbname=davical user=davical_app';
The only really essential thing there is that connect string for the database, although configuring someone for the admin e-mail is a really good idea.