The posted instructions are right but you still have a conflict. The problem is that ss files are detected as Scheme files by default and that setting is getting in the way of your user set setting. Therefore you need to get rid of that default, there isn't a nice way to do this inside N++ that I could find. I did find this post on stack overflow though:
http://superuser.com/questions/58672/how-to-set-the-default-language-in-notepad
The post by 'Mike Chess' gave me the answer:
In the Notepad++ directory look for the langs.xml file.
<Language name="scheme" ext="scm smd ss" commentLine="">
Delete the 'ss' and the one you set via N++'s GUI will work. Don't forget to save (and make sure to not edit it with N++...).
Restart Notepad++ Open a .ss file. It should be formatted as HTML now (if you did what was posted in the previous post by swaiba).
Cheers!