I don't really understand.....
Is your modem connected to usb or ethernet?
If it is an ethernet modem, then there should be no problem.

What you need to is run netcardconfig first, set a static IP. This is crucial because your networkcard is the lifeline of your system to your modem.
Unless your modem has a built-in router then you can choose DHCP, only if DHCP is set in that modem-router.
Then you run the pppoeconfig.