how do we make it show up at the sidebar ?
Topic on Extension talk:FeaturedFeeds/Flow
I would guess adding an external link to something like {{SERVER}}{{SCRIPTPATH}}/api.php?action=featuredfeed&feed=potd&feedformat=atom
to MediaWiki:Sidebar (see Manual:Interface/Sidebar) should work?
There is the $wgDisplayFeedsInSidebar configuration option that allow show feeds in the sidebar.