3. If they still ignore you (amazing how slow humans can be, huh?), make a big show out of finding your ball. The more ruckus you make in the process, the better.
4. They still haven't gotten the hint? Time to bring out the big guns: deposit said ball repeatedly on one of their body parts. Extremely effective if ball is covered in saliva.
Good luck training your human! Remember, they have very short attention spans (poor things), so keep your training sessions short but frequent. They'll get it eventually.