Cannot find /etc/apache2/sites-available/default when configuring Apache

The tutorial you are using is based on an older version of Ubuntu.

13.10 ships a newer apache configuration, where the file you are looking for is named /etc/apache2/sites-available/000-default.conf.

Note the .conf at the end, which is now required for apache to pick up on the files. So make sure the config you are creating for your site also has .conf as its extension.

/etc/apache2/sites-available/000-default.conf is the configuration file for the default site, only. You can add as many sites as you want, with their config files located in this same directory.

The apache global configuration file you're looking for is located in:

/etc/apache2/apache2.conf

As @drc mentioned, a lot of things changed in recent versions of Ubuntu.