Usually, the side menu in Pavilion is reserved for Category Menu Items with Template : With Pictures or List and the top menu is for all the other menu items.
You can, however, display a menu item other than the Category Menu Items if you add the "root-category-items" class to its element.
The way to do that is to go to Administration -> Nop-Templates -> Plugins -> Mega Menu -> edit your menu -> Menu Items and edit your menu item (in your case the custom link). You should put the root-category-items
class in the CSS classes setting. After saving the mega menu and reloading the public store your custom link should appear in the side menu.
Also please read the Nop Mega Menu Documentation
if any of my instructions aren`t clear to you. There you can find all the information on how to work with the Nop Mega Menu Administration.
Hope that helps!