OK, Assuming you're using Slackware 14.0
Your modem and connection are both DSL. Good. You don't need kppp.
Just use NetworkManager. To start using it, gain root in a terminal and just make the script for it executable like so:
chmod +x /etc/rc.d/rc.networkmanager
Next, do this:
sh /etc/rc.d/rc.networkmanager start
Then you should be able to use the networkmanager applet on the KDE panel to make your connections.
P.S., You're going to need an ethernet cable for this, not a telephone cable!!