As far as I know the search function doesn't work at all or only works sporadically.
For now, you can try this, it isn't as good I don't think, but it may help (the bolded parts are what you would type into the search bar on Google)
: Go to google.com, type in the search bar site:www.orthodoxchristianity.com/forum
and then whatever you want to search right after that. Example: site:www.orthodoxchristianity.net/forum true cross
(make sure and leave the space between the word forum and whatever you want to search for).
BTW, I just tried to search for true cross and some things came up that might interest you