Feeds are only available for public, non-passworded galleries.
On your SmugMug homepage, http://nickname.smugmug.com , you'll see feeds for recent photos, recent galleries, and popular photos (if your most popular photos are displayed on your homepage).
If your popular photos are hidden on your homepage, you can still find the feed here: http://nickname.smugmug.com/popular.
For your comments feed, login and go to your Account Settings. Click the Stats tab and then the "Details" button on the Comments line.
SmugMug's popular photos today: http://www.smugmug.com/popular/today .
SmugMug's popular photos of all-time: http://www.smugmug.com/popular/all .
SmugMug's popular photos for a certain category (example): http://www.smugmug.com/browse/Animals