"Bookmark URL's" seem to be a bit confusing thing - you need to enable it to be able to search for selection. And the addon doesn't work on this page, BTW
Bookmark URLs is extremely power tool and you can fully adapt it to your own needs. There are a lot of pre-configured URLs on the support site. And extension is working on this site once you will enable FF setting - see first item in the FAQ: https://bitbucket.org/ashemetov/selectionsk/wiki/FAQ.
