If you’ve imported video from your digital camera into iPhoto, it appears automatically in your iMovie Event Library (as long as the video is in a compatible format).
To use iPhoto video in iMovie:
Click iPhoto Videos in the Event Library.
The videos appear in the Event browser on the right. You can select and use the video just as you would any other footage.
To learn about importing video into iPhoto, open iPhoto and choose Help > iPhoto Help.