Scenario:
"I want to let people subscribe to my RSS feed but I can't tell them where it is!"
Solution:
Your RSS feed is at
http://yourdomain.com/whatever/xml-rss2.php
where "yourdomain.com" is the URL to your website and "whatever" is the directory you have installed Nucleus in (if you installed in the root directory, you will not need this).
Original forum thread (thanks, admun!):
http://forum.nucleuscms.org/viewtopic.php?p=37066#37066