ShareMeNot Options Rated 5 out of 5 stars
You access the ShareMeNot options via Tools > ShareMeNot > Manage Options, not through the add-on manager.
This info is available at the add-ons home page, as well as a FAQ and a test page to see if the add-on is working.
It's a great add-on, I've been looking for something like this. It doesn't completely block all sites, you can access the sites you want to use by clicking the button on a web site that has them.