1. Go to CMS Desk -> Content and choose Products in the content tree.
2. Click New at the document action toolbar.
3. Choose the Page(menu item) document type.
4. Enter Plasma TVs as the Page name, make sure the Use page template radio button is selected and choose E-commerce site -> Ecommerce - Products template. Click Save.
5. Your new page has been appended to the Products page. Now use the Down button at the document action toolbar to move it to the bottom of the section.
That's how you add a new category to your Products page.
Page url: http://devnet.kentico.com/docs/5_5r2/ecommerceguide/index.html?customizing_product_categories.htm