First off, if your modem connects to your PC via USB, you may be out of luck.. Everything that follows assumes that your modem connects to the PC via ethernet.
Assuming you haven't already gotten it working, here's the scoop:
Use Ameritech's Windows software to set your username and password, and verify that the connection is running correctly.
1) Download Roaring Pengiun's latest PPPOE client.
2) Follow instructions.
2.5) Your username is your e-mail address, INCLUDING '@ameritech.net'; you should set the DNS info to SERVER, meaning that the software will get the DNS info when you log in.
3) Check back here, and let us know how it turned out. Roaring Pengiun has forums on its own site, if you run into problems that we cannot fix here.