To capture a video clip - that is to download a segment of the video you have on your mini-DV tape onto the computer and store it there - you first should use the VCR-like control buttons to go to a point about three seconds before the beginning of the video segment you want to capture and stop there.
Then click on the Play button to begin playing the video.
Now click on the large Import button just above the Play button to begin capturing the video at that point.
The video you are capturing will play in the Monitor area and you'll also see an icon representing your captured video clip appear in the Shelf area to the right.
When you've captured all of this video clip that you want, click on the Import button again to stop the capture.
And stop the video by clicking on the Play button (or click on the Stop or Pause buttons).
You can then repeat this process and capture another clip by using the Play/Rewind/Fast Forward buttons to move to another segment of your video you want to capture.
At a point several seconds before that video clip, again start to play the video and then push the Import button. A new icon will appear in the Shelf area to the right for this second clip that you've captured. Cick on Import and then the Play button when you're done capturing this second clip.
Note: While you're capturing video, if there's a logical break in the footage - such as when you stopped recording with the camera off after shooting one scene and then turned record back on again to shoot a new scene - iMovie will automatically split that video into two clips in the Shelf area.