Countdown isn't a generator in FCP.
there's just too many variations of frame rates
there should be in your install disks a small collection of countdowns
it might be in a Cinema Tools Extras folder... or just Extras. (cant remember)
the cool thing about these is that there is also a Countdown generating Project,
where you can modify existing sequences to create your own custom one.
these countdowns are a lot less horrific than the edit to tape countdown, which is embarrassingly bad.
oh, and contrary to what i said before, they DO seem to be partially created with a custom generator, that i couldn't get to appear in the FCP generators.
another way is to just make one yourself.
if you don't need anything fancy or wipes between the numbers, it should take about 2 minutes.
cheers,
nick