The menu is not edited through the CMS, you need to edit the code on the webassist/plugins/main_menu/menu.php file directly to change the top level menu items.
the products that are displayed on the index page are the featured product and top selling product. Right now it is showing dummy data, this will change once you add products to the product catalog.
When adding a product in the Powerstore Admin section, there is a button "Feature ON Home Page" to control which item is featured.
until you get some orders, the featured item will also be the most popular item. once some orders are placed, the most popular will be the item that is ordered the most.