I'm basing this on Debian use, and since (K)Ubuntu is Debian based, the stuff should hopefully be the same or similar.
You'll want to be sure that the user in question is a member of both the div
groups. To check this, use the command: groups <username>
Here's an example:
$ groups rbochan
rbochan : rbochan cdrom audio src
If the user isn't a a member, you'll need to add the user to those groups using the command: adduser <username> <groupname>
as root, or using sudo in (K)Ubuntu. The user will have to log out and back in for the changes to take effect.
Other than that, there's a file in /etc/ppp (or a subdirectory of that) with kppp settings that includes the line:
Using your favorite text editor (as root, sudo, yada yada), remove the # from the beginning of that line. You might have to restart the ppp service, via the command /etc/init.d/ppp restart
before the changes take effect. The user should then be able to set up and use kppp.