Your external modem "should" either be at /dev/ttyS0 (COM1) or /dev/ttyS1 (COM2). kppp is probably using /dev/modem.

Try to set up kppp with /dev/ttyS1 as your modem and see if it works, if it does OK if not try /dev/ttyS0 as your modem device.

HTH

rock