I don't use treats or bribery. I take away any other water source, wait an hour or two so they get thirsty, then pick up each chicken (if you have a lot, a few of the more dominant birds will do) and use their beaks to toggle the nipple. Let them drink a couple drops, do it again, then switch to another bird.
You can also use your finger to peck at the nipples if the birds are all standing around watching, some will pick it up that way.
I don't permanently keep the alternate water source out until I see all the chickens in training drinking from the nipple, but to encourage them to use the nipple waterer I'd only bring it for a short period after several hours/at end of day.