To 'Add an Item to a Menu', from the store dashboard go to:
> Appearance > Menus
From within this page at the top, click the drop-down box next to where it says 'Select a menu to edit' and find the menu you'd like to make changes to, then click 'Select' to edit that specific menu on your store:
To add a specific 'product category', open up its corresponding drop-down on the left hand side (for 'Pages', open 'Pages', for 'Posts', open 'Posts' etc.):
To add a 'Product Category' or 'Product Categories', simply check the tick-boxes of the ones you'd like and click 'Add to Menu':
Your 'Product Categories' will then appear at the bottom of the 'Menu structure' over to the right hand side:
To rearrange the menus contents, simply drag and drop a menu item wherever you like.
To delete a menu item, click the arrow to right of it and then click 'Remove':
(*Once you're satisfied with the arrangement, don't forget to click 'Save Menu' at the top right*)