Drew can correct me if I'm wrong, but I believe the new WebsiteBuilder develops a sitemap automatically. I recently ran an SEO evaluation on my site right after I'd published it that indicated it already had a sitemap (which I had not added).
SD is correct - it is added automatically. To check it you can type your site address into the browser and it should come up. yoursitename.homesteadcloud.com/sitemap.xml
You can also add a Site Map page. This can be beneficial for both the Search Engines and visitors if your site has lots of pages with drop-downs. Just makes it easier for visitors to see everything at once. List the pages with links to them and you can add a snippet of info, a contact form, etc.