Just got my Arch Linux XFCE box up, customized & running. One thing I missed most was ‘vertical scrolling’ with the touchpad. So, I went through the great knowledge base of Arch Linux wiki and finally found out what’s needed.

First install the synaptics driver:

pacman -S xf86-input-synaptics  

Then install the GUI app:

pacman -S gsynaptics  

You should now find it under Applications Menu > Settings > Mouse and Touchpad.