Drop zones do not work from Photos browser
If we try to add images from the Photo Browser and the Apple Clip button is grayed out is because we can not do that in Final Cut Pro X yet.
The best way to do this is to import images directly to our Library's Event folder. Also, we can drag images from the Photos Browser in Final Cut Pro X directly to our events folder.
If you have any questions, please do not hesitate to ask. Have a great day!