LEMP on CentOS 6 returns 404 errors

I have created a CentOS 6.5 x64 droplet with LEMP installed as described here: https://www.digitalocean.com/community/articles/how-to-install-linux-nginx-mysql-php-lemp-stack-on-centos-6

It is working fine, http://vps.eletter.gr/info.php. I have ported a sample CodeIgniter application (https://github.com/scoumbourdis/grocery-crud/archive/v1.4.1.zip) under http://vps.eletter.gr/gc141/ but when I try to click to any of the links it returns 404 error.

I also tried to install FUEL CMS http://vps.eletter.gr/fuel/install but again it comes up with a 404 error where when on http://vps.eletter.gr/fuel works fine.

Obviously it has to do with the nginx conf file but I have tried many options without luck!

The /etc/nginx/conf.d/default.conf file is:

server { listen 80; server_name vps.eletter.gr;

location / {
    root   /usr/share/nginx/html;
    index index.php  index.html index.htm;
}

error_page  404              /404.html;
location = /404.html {
    root   /usr/share/nginx/html;
}

error_page   500 502 503 504  /50x.html;
location = /50x.html {
    root   /usr/share/nginx/html;
}

# pass the PHP scripts to FastCGI server 
#
location ~ \.php$ {
    root           /usr/share/nginx/html;
    fastcgi_pass    unix:/tmp/php5-fpm.sock;
    fastcgi_index  index.php;
    fastcgi_param  SCRIPT_FILENAME   $document_root$fastcgi_script_name;
    include        fastcgi_params;
}

}