If you think about it, there's not much to an "FAQ extension". You can implement it as a page type, categorize it through the "tagfield" module, rate each FAQ entry through a simple jQuery rating plugin, use the built-in silverstripe search, ...
I am also searching for a knowledge base or FAQ module. PHPKB from http://www.knowledgebase-script.com looks very promising. Is there a guide on how to integrate 3rd party applications such as PHPKB in SilverStripe?
There is not a direct guide on how to intergrate an external php script/class into silverstripe - the best advice I can give you is to follow the tutorials (possibly buy the book and work through that) and then follow Ingo's previous post. The tutorials and book (which is like a large detailed tutorial) are actually fairly fast.
Alternatively you could recruit an experienced silverstirpe developer to make it for you.
Thanks for the quick response. I am afraid I don't have much spare time to go through the book so I am thinking of contacting the guys at http://www.knowledgebase-script.com to see if they have any experience in integration of their knowledge base software with SilverStripe.
I doubt you'll have some luck there as I've now browsed the site and it's proprietry software - you'd be better off having your silverstripe site and linking to this maybe a subdomain of your main site. Either best of luck!