When I introduced my big buckets with nipple waterers on bottom in the first place, my chickens got the hang of it after a couple days. I just had them watch while I toggled a nipple with my finger, and after a few minutes, they just picked up on it.
I lost Potsy to misunderstanding the watering system (which is odd, because she'd had it in the house and used it with no problems) and nearly lost Fonzie.
I was determined to really watch the new chicks and make sure they got the concept down. They didn't. It was maddening. I could tell they were thirsty. I would use my finger to make water come out of the bottle, but they didn't get it. After a day or so, they got all excited when they saw my finger, as they saw it, it was the magical water dispensing finger. Yesterday, at wits end, I ended up squeezing the water bottle. It is soft enough that it forced water to slowly drip out. They saw that, and a light bulb went on. Within a couple minutes, everyone was drinking from the nipple, and all was well.
I've heard of adult chickens continuing to have a problem, and my fix won't work if it's a 5 gallon bucket. I haven't tried this, but it makes sense. Someone suggested using grapes or cherry tomatoes, and carefully sticking them over the nipples. The chickens will nibble at their treat, and by the time the treat is gone, they'll have realized there's water coming out of that strange, silver contraption.