Default Alignment
Out of the box, your submenus will be vertically centered to their parent menu items. That means there will be an equal amount of submenu above and below the top level menu item, without exceeding the bounds of the top end of the menu bar. This ensures that the parent item and submenu areas are adjacent, so that the submenu will be accessible on hover.
If your submenus are not centering as described above, you likely have a javascript error that needs to be resolved. Lack of animated submenu reveal is also an indication.
Align to the top of the menu bar
If you want to align a submenu to the top of the menu bar, regardless of the position of its parent item, enable the Full Width Submenu option on the top level menu item.
Make sure you only do this if your submenu is tall enough to reach the parent menu item in the menu bar, otherwise it will not be accessible.
Align to top of parent item
If you want to align the submenus to their parent items, rather than relative to the menu bar, add this CSS:
#megaMenu.megaMenuVertical ul.megaMenu li.menu-item.ss-nav-menu-item-depth-0{
position:relative;
}
#megaMenu.megaMenuVertical ul.megaMenu li.menu-item.ss-nav-menu-item-depth-0 > ul.sub-menu{
top:0 !important;
}