I also had problems connecting to the internet with the Optus Siemens Speedstream 4200 ADSL modem and the Airport Extreme base station.
Apple Technical Support were great with helping to solve it- they are well aware of the problem apparently:
The Speedstream 4200 modem supplied by Optus comes pre-configured in a mode called "Optus Bridge", and the optus PPPoE username/password are stored in the server in the modem.
For the modem to work with airport extreme, you will need to change the modem setting to "bridge" mode, and then configure airport extreme to connect using PPPoE
To change modem to bridge mode:
Connect modem directly to computer by ethernet.
Access server in modem by typing
http://10.1.1.1 into a web-browser (NB Safari 3 can be temperamental - I used Firefox)
Click on settings on the left hand menu
Click on the "mode"
Go to settings and change mode to "Bridge".
Save settings
Reboot modem.
Once the modem has rebooted completely, turn off power and connect modem to WAN port of airport extreme.
Now use the Airport Utility to configure the airport extreme base station:
(AirPort Utility is located in the Utilities folder in the Applications folder on a Mac, and in Start > All Programs > AirPort on a computer using Windows)
Open AirPort Utility, select your base station, and then choose Manual Setup from the Base Station menu. Enter the base station password if necessary. Click Internet in the toolbar, and then click Internet Connection. Choose PPPoE from the Connect Using pop-up menu.
To set up your base station to use PPPoE:
Enter the User Name, Password, and any other information your ISP gave you.
Click Update to reset the base station with these new settings.
Turn on your modem once the base station has reset.
You should now be able to connect to the internet
On a windows computer, you may need to reboot the computer.
Hope this helps.