If your ISP requires PPPoE for authentication prior to gaining accessing to their Internet service, only the device directly connected to their modem would required to be configured to provide the PPPoE credentials (i.e., username & password).
If your AirPort Extreme is asking for these credentials, then it may believe that it is the device that will be sending these credentials to the ISP.
You mention DSL. A number of DSL ISPs do require PPPoE. What is the exact make & model of the DSL modem that Quest has provided you?
If the set up with the Qwest is a new one, then I would recommend that you perform a complete power recycle of your networking hardware to get things communicating properly. Please check out the following AirPort User Tip for details.