Some dogs take to standing on the treadmill right away, while others may become nervous if they can feel the platform wobbling slightly. If your dog is hesitant about standing on the treadmill, use a pleasant scent or a treat to lead them onto it and repeat until your dog is no longer nervous, using the pleasant stimulus to create a positive association.
Once your dog is used to standing on the treadmill calmly, then you can start it out at its lowest speed. Most dogs will naturally begin walking, although you should have your dog attached by a short lead to the front of the treadmill at this point.
You would be amazed at how quickly a lot of dogs take to the idea once the treadmill is in motion.