Did you also tried to configure the modem and compile the kernel modules ? Since that's also quite an important step. No kernel modules means no way for kppp to detect the device. Since the above knowledge also came from some other site I'm now going to refer to that site for more instructions :http://xlife.zuavra.net/columns/20021104/
They also offer information on how to configure the modem from the commandline, but I don't know how comfortable you are with that (powerful) tool.
ps. If you need more help, or there are things you don't understand in the column, feel free to post again.