You need to create a RSS pagetype and configure it for your feed.
Go to your app Dashboard -> Edit Pages -> Click + button at the bottom of the Pages list -> Choose name of your page, RSS type and click Create.
Then select your page in the list and click Edit. Copy your RSS feed url to RSS URL field. Then click Apply to preview or Save.
You need to link to this RSS page from one of your root pages (tab1.html - tab4.html, or index.html depending on which template you use). Or you can convert a tab (e.g. tab2.html) into RSS type and then configure it. To convert a page to a different type, go to Edit page and click Convert just below it's title.