Firstly, download this file.

1. Copy/upload smoothscroll.js and jquery.nav.js to /templates/TEMPLATE_NAME/js/
2. Open main.js in the same directory as above
3. Copy the Content from main.js.txt to main.js and paste before "// go to top" (Line 39)
4. Edit helixultimate.php file in /plugins/system/helixultimate/core/ as described in helixulitmate.php.txt
5. Done