I have three coops with only nipple waterers. Put the waterer your using near the one your training them to with the nipples. They'll peck at the nipples and realize what they are, mine are red one's might help. Once you see they are pecking at and drinking from the nipples take the other water away.
Sooo much easier and cleaner, I barely ever have to fill them up.
Good luck, I think you'll be pleased once they get the hang of it, should only take a day or two.