The Groovy Mega Menu Plugin is fully compatible with the DIVI Theme Builder. Integration is occur automatically immediately after you activate the plugin, you only need to add and configure the header for that 

Go to DIVI > Theme Builder. Then please click on Add global header 

How to add mega menu in DIVI Theme Builder

Then Build a global header > Build from scratch > Start Building > Insert Row 

Depending on the selected Row type i.e. Fullwidth or Regular, the appropriate module will be available to сhoise.

Groovy Menu Regular module  (Boxed)

or Groovy menu Fullwidth module 

Then you can configure the module properties. Add paddings, set the width and others

After that you can start building the mega menu block

You can add elements such as mega menu: Accordeon, Counters, Blog posts, Buttons, Images, Call to Action, Custom code, Contact form, Map, Login area, Member Area, Slider, Shop posts, Video, Tabs and others 

Learn mode from video tutorial