Turn on the faucet all the way and press on the lever to show your dog how it works. Your dog will see the water come out and start drinking.
If your dog does not start drinking immediately, help your dog learn by unscrewing the waterer from the faucet just enough so water trickles from the threads down over the waterer. This should attract your dog. Be sure to remove other sources of water. After several hours, demonstrate for your dog again. You can also smear some peanut butter or soft cheese on the lever to encourage your dog to lick it and activate the waterer. Once your dog is trained, be sure to tighten the waterer onto the faucet.