This happens a lot. A lot of people including myself do not have the time to teach every puppy how to work the mill. The first thing that scares a dog about the mill is that it is in a house or basement in which the dog has never seen or has got a chance to get use to. The second thing is that the mill is high up off the ground, and it makes a lot of noise. I use to hear guys say "hurry up and grab him off, you are teaching him thats it's ok to just stand there". LOL Boy were they wrong!

Let him run the area where the mill is, he may even walk up and lift his leg to piss on it. Do that for a couple days. Then every now and then set him up on it and then pull him off and praise him. I used hot dogs. Eveytime the dog would take a few steps I would pull him off and give him a little bit of hot dog, and after a while the dog was trying to jump on the mill himself. It only took me about a week and I had him running it fast. After that I just used a chicken. Once they get over the whole scared part the chicken will have them running like a raped ape.
I would also grab one and put him on the mill and let him stand there while I fed the other dogs and cleaned up poop, then I would come back to him and let him eat his dinner right up on the mill. After a while he was use to being up there, then I would just use hide or what ever to get him to run.

The very best way to get them to run a slatmill is to buy a carpet mill, This is lower to the goung and much more quiet, and after they run that for a while they will do the same on a slatmill.