From your dashboard, click the menu item "Rental Products" > "Products" > click the "Add Product" button.
1) If this is your first product, click the "New Category" radio button. Name the category in which you want to add this new product.
2) Add the "Name", "Image" (square or close-to-square images will display best), "Size" of the unit, number of "Units in Stock" and product "Description".
3) Add the "Price Option" and associated "Price"
Click the "Create Product" button and you are done.
You can now go see this product on you website. Being doing so, you may also want to add an image for the new category you just created. Do this by going to "Rental Products" > "Categories" and click the pencil (edit) icon in the "Actions" column.