There are 2 types of menu structures in Erickson, the default is a simple dropdown menu that uses an accordian style open/close on click design and the second is a Mega menu.
Learn how to create a dropdown menu with nested links here.
Go to Online Store → Navigation → Add menu
By default, your will see 2 menus, Main menu and Footer - click on either to start editing the current menus
To add a new Mega menu, go to Online Store → Themes → Header
Header → Add Mega menu