If you find that you want to add just a few frames to the beginning or end of your clip, Fine Tuning makes this quite a bit easier. In the bottom corners of a clip, there is a button with a line that has two arrows on either side. You can see the buttons I am talking about in the bottom corners of this clip.
When you click on either button, an orange border wraps around that end of the clip. This border comes with a draggable handle.
Go ahead and drag the handle. As you do, you will see iMovie counting the frames that you add or remove from your movie, together with the Viewer window showing the footage you are adding. Pretty dang handy.
One other tip: if you hold down Command-Option as you mouse around in your project, you will see the Fine Tuning option pop up without the need to click the little buttons.