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.