With iPhoto you can drag the videos to your desktop and then drag them into iMovie.
Or, if iPhoto appears in the libraries list in the iMovie media sidebar you can click on the iPhoto item and access your videos from iMovie that way.
The same procedures above described would work for the newer Photos app as well, except that you can also drag directly from the Photos app library into iMovie. To access your videos from the iMovie Photos app item in the sidebar, the Photos library must be designated as the system photo library in the Photos app General Preferences.
-- Rich