Smooth Scroll Main Menu Anchor Links for One-Page Website

How do I get main menu same-page anchor navigation links to work with smooth scrolling? I’ve tried exposing the CSS Classes option in custom link menu properties and inserted “smooth-scroll” and saved.

https://52.70.39.122/wp-content/uploads/2016/01/Screen-Shot-2016-01-13-at-8.59.25-PM.png

However it doesn’t work. In fact it stops the anchor from working entirely. It works without that attribute but then goes straight to that location on the page rather than smooth scrolling to that location. You can see the site in progress here and click on the “about” link in the main menu to see what I’m talking about.

https://52.70.39.122

How do I get smooth scrolling to work with anchors in the main menu on the same page?

Thanks in advance.