How do I add new sites to be blocked?

[mingusmoonwalk]

I want to also block the site "instructables.com" (for example).

When I extract the downloaded zip-File in Ubuntu, I get these files:
get-shit-done
get-shit-done.php
get-shit-done.py
get-shit-done.sh
README.md
sites.ini

So I figured that I just have to open sites.ini with gedit and add "instructables.com" to block it. However, this does not work. So what do I have to do instead?

Thanks.