The first thing to do is make sure that pppd is setuid root. This means that no matter what user executes pppd it will run as root. If you type "ls -l /usr/sbin/pppd" you should see something like
-rwsr-xr-x 1 root root 95225 Jul 11 00:27 /usr/sbin/pppd
If it is not setuid root type the following command (as root):
chmod u+s /usr/sbin/pppd
Also, I THINK kppp uses an access file (/etc/kppp.allow if I recall correctly). Make sure the username you are trying to dial in as is in this file.