You can easily add, delete, and re-order items in the site navigation. Add items to the navigation by going to Entries > Site Navigation and adding a new entry.
If you fill out the Nav Header field do not use the nav link.
The Nav Link field has two options: Entry or Url. Use url for an offsite link and use entry for any page on the site. When using Entry the navigation will use the Entry's title unless you fill out the Link Text Option.
The + icon will appear on top level navigation items automatically when they have sub items below them.