We bought a similar feeder at the RWAS show this year a few weeks back.
It's got three settings:
1. To get them used to the feeder the treadle sits on the ground with the lid open. So no movement in the treadle.
2. To get them used to the treadle, the intermediate setting has the lid half-open and the treadle has a small amount of movement.
3. Once they've got the hang of that it's closed, with it only opening when a hen stands on the treadle.
This is all explained here:
http://www.grandpasfeeders.co.uk/how-it-worksIt's controlled using screws that stop the levers at certain points. I can't see from your photo exactly how your's works, but you could probably rig something similar up?