If your Mac is currently cabled to DSL modem provided by Verizon and you now want to use the Airport Base Station, the Airport Utility will walk you through the process of setting up the Base Station so that it uses the PPPoE connection protocol required by Verizon.
If you want to know how to manually set up your Base Station for PPPoE, start reading at page 25 of the manual "Designing Airport Networks" found at:
http://manuals.info.apple.com/en/DesigningAirPort_Networks_Using_AirPortUtility.pdf
What the manual doesn't tell you is that - after you have finished configuring the Base Station and saved settings to it - you should pull power to your DSL modem and the Base Station for a few minutes. Then plug the modem into power and wait a minute, then plug the base station into power.