Click on 'menu' and select the menu to which you want to add categories and products. Then click on the green 'add category' button and select the categories you wish to add. Be sure to click 'save' in the upper right corner once you have selected all the categories.
To add a product to a category, go to 'menu' and under each category, you'll find the phrase with a green plus sign: 'add product to category'. Clicking on this will display your product list. You can scroll through and select products or use the search function for convenience. Once you have added all the products, click on the green 'add products' button.
Once all the products have been added, click 'save' in the upper right corner to save your changes.
