Select the Category you want to copy from to populate the Checkbox dropdown. Then select the checkbox you want to add to the category you are editing. You also have to choose to either move the item across (it will be removed from the original category) or copy it across (it will still appear int he original category)

Click Save Category button and the item you selected will be added to this Category at the end of the category. You can now drag and drop it if you want it to appear in a different order.