How to use(/enable) FN keys to adjust volume at my asus laptop?

I had the same problem with my ASUS K53E.

sudo apt-get install git

git clone git://git.iksaif.net/acpi4asus-dkms.git

cd acpi4asus-dkms 

make 

sudo make install

sudo modprobe asus-nb-wmi  

After every kernel update the above will be erased and you will need to re-do it every time until it is included in a kernel version at a later stage. To make it easier create a .deb package which will be easier to install.

sudo apt-get install debhelper 

cd acpi4asus-dkms 

dpkg-buildpackage

sudo dpkg -i [newly-created-package].deb

sudo modprobe asus-nb-wmi

source

I have a little different computer though; mine is a K53E and yours is an X53S, so I can't guarantee that it'll work.