I found some info here, but I guess it's related to the old version...
did you try searching for comment sort in the search tab. just wondering?
this won't help huh?
personally, I think its a bad idea because it promotes people discussing the last comment without reading what posters have already surmised or suggested in the discussion already which leads to alot of duplication. But to each his own. Why not just have them automatically go to the last comment at the bottom and scroll up. (or provide the option)
I may not provide the completed solution you might desire, but I do try to provide honest suggestions to help you solve your issue.