I had this same problem and I just resolved it using the feedback provided in this thread. I had some difficulty following the thread though, so I am going to post clear instructions here.
The Problem:
While trying to import files from Photos to iMovie, the option to import files from Photos is greyed out. Any attempt to drag and drop files out of Photos and into another file does not work.
The Solution:
- In iMovie, select the "Project Media" tab at the top.
- In the left pane under "Libraries" select "Photos Libraries," which has a black and white version of the Photos icon.
- Now along the top of the "Project Media" area, you should see a drop down menu with various items in it (Shared, Places, Moments, Collections, Years, Albums, and My Albums). Select Years to see all files. Note: at first this drop down menu did not give me all of these options. It only gave me the option to select "shared." After I restarted iMovie, I was able to select the "Years" option to see all of my files in Photos.
If this Solution does not work:
- In Photos, select Preferences.
- Within Preferences, click "Use as System Photo Library" button.
- Restart iMovie and try the above solution.