When I tried to subscribe to http://arstechnica.com/security/, it was not one of the available choices. I don’t want to subscribe to all of Ars, just the security section.
You just need to get the rss feed for that site. They are all listed here: http://arstechnica.com/rss-feeds/
Thanks. Your recommendation worked!! So the problem really was that NewsBlur did not detect all of the sub-feeds from Ars Technica when I typed Ars Technica in the “+” box. When I did as you said, I was able to add the specific subsection I was looking for.