I can see the videos in my Photos Library in iMovie, like this while viewing a project:
- Select the Photos Library in the sidebar with the libraries. Wait.
- Set the selector to "Albums" and select one of my albums, that only contains videos:
This will let me drag video clips to the timeline.
Is your Photos Library stored locally or an iCloud Photo Library? If the videos are only in iCloud, select the videos you want to use in iMovie and use "File > Export > Export original ..." to export your videos to a folder on the Desktop. Then add them from there to your iMovie project.
Dragging and dropping from Photos will not work, because dragging will just export a preview of the edited version. It reduces the quality of your videos.
Added: The menu to the left of the search field should be set to "All" or "Videos", not to photos. Toggle it to "videos", to make sure it is not excluding the videos.
And if your Photos Library does not appear at all in iMovie, check the Photos > Preferences > General. The option "Use as System Photo Library" needs to be greyed out, to indicate that that it is your current system photo library and other applications can access it.
If you have to change this option, quit Photos and quit iMovie after you enabled it.
