Mountain Lion no longer supports RSS feeds in either Mail or Safari, but John Carter to the rescue. He offers this description and solution: "If you are currently subscribed to either the PMUG website RSS or the Newsletter RSS, you should consider one of the many alternatives for receiving RSS feeds.
"I use NetNewsWire, the free version. It works, but not quite the way I would like it to. When I was subscribing to RSS feeds with Mail, I could delete news that I didn't care to keep. You can't do that in NetNewsWire, at least not in the free version.
"NetNewsWire also automatically deletes old news. It seems the only way to hang onto a copy of any news item is to display it, copy it, and paste it into a document. But then, you can still go to the site that delivered the news feed and retrieve any material there by searching for the topic. Now the funny part of this is for me, I rarely go back and reread any news. I do at times refer to it to pass on information that others are asking about.
"To subscribe to an RSS feed in Mountain Lion, you first have to have an RSS news reader installed on your computer. Next, open Safari and go to the website that you want to subscribe to their RSS feed. Click on the RSS icon. It opens NetNewsWire (if that's what you have installed) and pops up a dialogue box. In the dialogue box, enter a decent short title and a description (optional).
"If you are still using Leopard, Snow Leopard, or Lion and you want to start right away with an RSS reader like NetNewsWire, open Mail -> Preferences and click on the RSS tab. There you will find an option to use Mail or an external reader. Click on the option to use an external reader, then in the Finder window that opens, find and click on the RSS reader, then click OK. Any new RSS feeds that come in will go to that reader. You can now delete the RSS feed in Mail. If you're using Safari as your RSS reader, do pretty much the same thing there in Safari Preferences.
"If you choose to use NetNewsWire, when you click on a news item you will get the short version. To read the long version, you can either double-click on the entry or click on the title of the news feed - this appears if large blue letters just above the short version."
(Click to enlarge the illustrations.)
John winds up his instructions with, "In NetNewsWire preferences you select to open the long version in NewNewsWire (in a separate tab) or in your default browser."