Hi,
Unfortunately, iMovie 10 doesn't have a countdown timer feature.
There is a time display above the timeline for the hours, minutes, and seconds in your project and the elapsed time at the point where the play head is located.

Within a clip you can also display the hh:mm:ss:ff (frames) by holding down the "r" key and dragging the cursor across the clip while pressing down on the trackpad. But that's it.

However, there are count down timers on the internet and You Tube that you can download (respect copyrights, though) and insert into your movie by using the Picture in Picture feature. You can also create your own count down timer by taking a video of the count down timer on your iPhone and inserting that into your iMovie project video.
Take a look at this thread for some interesting ideas about how to insert a timer into iMovie:
Insert a timer in iMovie 11
-- Rich