Do you mean for the forums? I don’t have very good luck with the built in search.
There is a search link up at the top, under the line “The open-source media server for Roku Soundbridge and iTunes”. But the search in phpBB doesn’t work very well.
It is better to use Google to search the site. Just put “site:fireflymediaserver.org” (without the quotes, of course) in front of your search phrase in Google, and it will restrict its searching to this site only. That way, you can get results from the wiki, the forums, etc.