Feeds are only available for public, non-passworded galleries.
In New SmugMug
You can reach the feeds using the Share button and choose Get a Link. Then click Feed Links
Using the Feed Wizard, you can create several different types of feeds.
In Legacy SmugMug
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