Follow the steps on how to add a sub-category under a main category.
From your site's dashboard go to Listings > Listing Category
In the Categories page fill in the fields with the name and slug of your sub-category. In the parent field, click the downarrow and choose the category where you want your sub-category to go under. Click Add New Category button once you are done.
You will then notice the sub-category you just added is listed just below it's main category.