It might be a corrupt preference. Try opening iMovie while holding down the Option and Command keys and selecting to
delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if the download window will see the media in the Downloads folder.
If no luck, try redownloading iMovie. To update/redownload iMovie 10, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the trash. Sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar. You will see your purchases displayed, including iMovie 10. Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the trash and return it to the Applications folder.
As a workaround, you can try dragging the media out of the Downloads folder, and then into iMovie.
-- Rich