How to Import Videos into iMovie
You may have existing video that you want to use in your final blockbuster — video that's already on your Mac's hard drive or SSD. Perhaps it's a project you previously created in iMovie (the old version), or video from a digital still or camera phone that resides in iPhoto. Or maybe you want to incorporate video on a CD or DVD.
To import iMovie projects or other videos on your hard drive (or other disks) from Finder, select a file or cmd+click to select multiple files and drag them from Finder to the Event in the Libraries list in iMovie. By the way, these files need not be just video files; as you'll discover, you can grab photos and audio files, too.
As an alternative, you can import media by clicking the Import button on the toolbar and selecting content in the Devices section of the Sidebar. You can also summon the Import window by choosing File→Import Media.
Videos that you've already downloaded to your iPhoto library are readily accessible in iMovie. At the top of the iMovie Sidebar is a Libraries list, which is essentially the content library at your moviemaking disposal.
iMovie automatically supplies a library for iPhoto, and for Apple's Aperture as well, assuming that you own that program too. Merely click iPhoto Library in the list, and choose the Event you want. The only caveat is that the video must be in a compatible iMovie format.