I don't have that exact treadle feeder. However, the concept is the same.
I found I needed about 4 sizes of wood to hold the flap open in different stages. I used pieces of 2x4.
The first one held it mostly open, the next one 3/4 open, then 1/2 open, and so on. I used each size for 1-2 weeks until I was sure that they were using it.
So it was very gradual.
I've also heard that you want to remove other sources of food, however, if your chickens are molting, or under some other kind of stress right now, that might not be a good idea.
Thanks for the advice. I'll definitely try again with that method. I guess it wasn't gradual enough for them.
Some chickens catch on immediately while others will take days/weeks to finally trust the "new" contraption in the yard. Not feeding them open dishes at the same time you are trying to train them to use the treadle feeder is important. It's like training them to use nipple valve waterers -- can't have open dish water in the yard or they'll never learn to use the new nipple waterers. Some of my chickens were more afraid of the movement of the treadle lid than the noise it made. I have the Feed-O-Matic smallest feeder and the 20-lb ChickenCondos treadle feeder. I like the Feed-O-Matic for having a locking lid and plastic treadle foot for cleaning but most of my flock likes the bigger ChickenCondos feeder with the wooden treadle foot.
Feed-O-Matic - the Ameraucana would NEVER use the Feed-O-Matic and stayed back but the Blue Breda used it the very first day!
The Silkies seem to like the quieter ChickenCondos feeder - don't know why - chickens do their own thing.
Both my feeders open inward when the chicken steps on the treadle foot. Grandpa's Treadle Feeder has a lid that lifts up over the chicken's head and if another chicken sticks their head in when the other chicken steps off the treadle foot I was afraid the lid would crash down on the chicken's neck. Probably a paranoid worry -- but that was the reason I went with inward openers rather than lifting lid openers on the treadle feeders. I'm glad I got two different models of treadle feeders since the chickens pick their favorite to eat from.