Navigate to Configuration > Menus > Create a new item button.
Learn how to create a button by reading the Creating A Menu - Buttons.
Once your button is created, Click Browse under Item Options > Select the category you want to create > Choose if you want the buttons to be placed horizontal or vertical > Click Auto Generate
A new button for each item in that category will be created and placed on the menu.
TIP: Layout your menu with labels, size buttons etc. first. Then, auto generate the item buttons and they will fill in around using the available blank spots.
When you have finished making your buttons click the Update Now button to save your changes.