I could not find "Demo" in the link you supplied.
So i went to google and found this:
1) Make a domain called demo.yoursite.whatever
2) make the username & passwords demo
3) Go into WHM webhost manager.
4) Scroll down to Account Functions
5) Go to Disable or Enable Demo Mode
6) Choose the demo account you setup
7) Highlight the demo account and click
8) Enable demo mode
Then place a link to
http://demo.yoursite.whatever/cpanel on your site.
Its that simple.
except [ 5) Go to Disable or Enable Demo Mode ] doesn't show up
on our "WHM" panel...so know what ...lol