Categories not showing correct SLUG

Using Market Press for a client in Toowoomba AO Adult Store. The Categories have SLUG's and they are working correctly eg < https://www.aoadultstore.com.au/store/products/category/vibrators > however when I create a dropdown list of the categories on the web sites home page and a user clicks on an item from the dropdown menu the following URL is produced < https://www.aoadultstore.com.au/?product_category=8 > Basically what is happening it the SLUG is not being used in dropdown menu mode.