chap or pap

Hi, I have pap authentication working just fine. I want to change auth type to chap. I am using the radius book that is very outdated. Don't really understand why can't chap to work. I looked in the default file and chap auth-type is not commented out, so I am assuming that if pap authentication didn't work then chap would be the next auth type radius would try right. that is why they put pap at the bottom of the file to let the other auth types a try first. I am using mysql backend server with username and cleartext := passwords. Can anyone point me the right direction or tell me to read more:P jon