Tageting a specific menu link with nav_menu_css_class

An easier way to go about this: in menu administration, add your custom class directly to the menu item. To enable this, go to Screen Options at the top of the menu editing screen and check the “CSS Classes” box under “Show advanced menu properties.”

Once you check that box, you’ll have a new “CSS Classes (optional)” text input box for each menu item, and you can add it there.