"QuestWatchFrame" doesn't exist anymore. In 3.1 it was replaced by the "objectives frame"... I think this is named "ObjectivesFrame" but I could be wrong.
Like Arnath says, though, if you're more specific about what you're trying to do, there's probably a better solution than killing the entire objectives tracking system and trying to build your own from scratch.
Especially since the new frame tracks much more than quests, AND addons can resiter their own objectives to track. If you just want to replace quest tracking, you should look higher up... turn off quest tracking, untrack any quests that are tracked, and hijack the shift-click handlers in the quest log to turn on your own tracking craps.
If you go "replacing" the watch frame, you WILL break other addons that use the new stuff.
Well again, there's no reason to replace the frame in that situation. You can just remove the existing handler for quests and add a new one (check out the WatchFrame_AddObjectiveHandler function in WatchFrame.lua).
EDIT: In case it wasn't clear, the new tracker frame is called the "WatchFrame".