Step 1 : Log into your WordPress Dashboard and navigate to Appearance > Menus
Step 2 : Blokco theme comes with mega menu creation plugin built in. See below image for its decription to see how it works
Check the mega menu option in any top level menu item and choose the VC section for the mega menu content. VC sections can be created using the Visual Composer page builder.
Step 3 : Select Create a new menu at the top of the page.
Step 4 : Enter a name for your new menu in the Menu Name box and click create menu.
Step 5 : Your menu is now defined, and you can now add pages to it.