First you need to identify where is the kppp program with the command
It will display the directory where kppp is installed, for example, /usr/lib/kde3/bin/kppp; then enter the command
ls -l /usr/lib/kde3/bin/kppp
It will display the attributes of the file
-rwsr-xr-x 1 root root 543096 2005-07-25 10:50 /usr/lib/kde3/bin/kppp
If the first permission letters aren't "rws" or the user/group are not root as shown on the above display, this is your problem; In this case, as root enter the commands
chown root.root /usr/lib/kde3/bin/kppp
chmod u+s /usr/lib/kde3/kppp
Remember to replace the "/usr/lib/kde3" by the directory you found on the first command in this post.
The kppp must be setuid; it must always run as root.