when you start up kppp you are prompted for your username and password (before you dial in).
Does your internet connection function? As in does it connect, or do you get disconnected / does pppd return an error, is there an actual error message?
if you get a pppd error then because authorization could not take place, check the pppd options in the kppp setup, and add the noauth setting. You will need to be root to use kppp. If you recon that's the problem, let me know here and we'll help you (further).