To link to a section you create a link that looks like this: #sectionname
If you are going to place this menu on the same page as the section you want to link to, you only need to type the #sectionname. If you want to link to a section on a page other than the one your menu is on you need to type the full URL of that page with the #sectionname at the end like this:
350.org/examplepage/#sectionname.
For this example we are placing the menu on the same page as the section we are linking to. Add the URL and Link Text as shown below then click Add To Menu. When you have added all the links you need to add click "Save Menu".