Not having used this script before I will be running blind here. I will try to spell out what steps I would go through based on the information available and hope that addresses your needs.
Open Terminal from the Xubuntu Applications Menu>Accessories>Terminal
From what I can see the id you are after is 13, with that in mind you would execute. (# is a prompt in the terminal)
# chmod a+x enable-rightbutton.sh
# ./enable-rightbutton.sh 13
If it needs to be run an Xorg startup you should be able to place a call to execute the script in an .xprofile file located in your home directory
Create the ~/.xprofile file
# touch ~/.xprofile
# gedit ~/.xprofile
Add the following line to the .xprofile file and save it
/<absolute path to file>/enable-rightbutton.sh 13
I use this .xprofile trick to set my resolutions through xrandr, control volumes/muting of devices, etc at xorg startup.
Not come across this issue myself, so i'm working on the assumption that you were after how best to implement the script.