Success! After giving up several times, I've finally got the chooks to use their fancy chooketeria feeder (which cost over $100!)
I wanted them to use it so I could have their food outside the coop, without rain, mice or wild birds getting into it. I tried having it propped open halfway, and they would stand at the side and eat out of it. If they stood on the step they would get a fright and run away when it made a noise.
Finally I decided that they were just not hungry enough, and put their food only in the chooketeria - with a bolt in the side stopping it from closing all the way, so they could see the food. It took several days but finally the bravest one did start holding it open by stepping on it. The Light Sussex took advantage of the other chooks opening the feeder, though they often chased her away. After they had been using it this way for a few weeks I took the bolts out and again it took a few days for them to become brave enough to open it. Now they all happily feed from the chooketeria, but the white chook who has never been very brave won't open it herself.
Checking out the chooketeria
Chooketeria propped open