I know there's a way to do this, but I cannot for the life of me figure out how to assign a product to a category after it has been created in the TOP PRODUCTS.
 
Brain freeze or something. :-}

select top category, then "categories" is an option beneath each of the products. Click and follow menus.
 

All I see under each product is:
 
Prices | Images | Options & Components | Specification | Related | | Accessories | Releases
 
I think something might be missing??

it's normally between Related and Accessories
 
you could always export it, copy the correct catagory from another product and import it back

You clearly have a missing item - * = categories:
| Related |*| Accessories |
 
Don't mess with a fix - I would ask Viart - submit a ticket as this is an error that they will correct for you.
 
Chris