At the moment, all you can do is use this makeshift forum search.
Or, you could use the very basic keyword search that somehow found its way into the coding throughout the years.
http://armorgames.com/search/keyword/community
But replace keyword with what you want to search for. It only works for single words, but it finds every thread ever made that contains that word. It's not incredibly useful, though, unless you have a good keyword that doesn't generate thousands of results.
However, the idea of a working forum search has been suggested on multiple occasions and it's very likely that one will exist in the future. Any future suggestions you have for the forums should go in this thread.