Be sure to substitute in the correct blog address forblognameand the label you're interested in forlabelname. Also, don't miss the hyphen ("-") in the URL. That's not a typo! Note that a URL of this form will only work if the blog in question is using this specific label, and has also enabled site feeds. However, no special site feed settings are necessary. As long as the basic site feed for posts is enabled (with either short or full descriptions) then label feeds will also work.