Go to Appearance > + Xtreme Plus > Extra Menu, and explore the options, you have the choice of the type (standard , accordion…) you probably want standard and then a style, Primary / secondary / Extra to use one of the options sets available in main options menus.
So a menu bar filled with the item of a custom menu named mymenu and using the primary style would be
[extra_menu menu_name='mymenu' menu_style='primary' menu_type='standard']
You may need custom CSS on the Fixed top area, but we can deal with that once in place 🙂