Where is the Apache configuration located on Linux Mint?
On Debian-based systems (which Mint is), the Apache configuration takes a different approach. Every "site" has its own configuration in /etc/apache2/sites-available.
The default configuration is named default. So, try:
gedit /etc/apache2/sites-available/default
Don't forget to reload or restart Apache afterwards:
sudo invoke-rc.d apache2 restart
As a sidenote, you can create additional configuration files in /etc/apache2/sites-available and activate them using:
sudo a2ensite <yourconfig>
You can disable the same configuration with:
sudo a2dissite <yourconfig>