When the add-on is installed, the settings page should automatically appear. If not, you can try reinstalling it. Alternatively, you can get to the settings by pressing the Firefox menu button, selecting add-ons, picking the Custom Scrollbars add-on and press the options tab.

If you still need help, email me at [email protected] and I'd be happy to help. It sounds like the add-on would be able to solve the problem you are having.