BellSouth ADSL uses PPPoE for connecting. There will be a setting in the router for the type of connection. If you choose PPPoE, the router will have an area to input the username and password (generally the same as the main email account) such as email@example.com/MyPassword.
The Westell modem is also a router. If using the Linksys to assign DHCP addresses, the DHCP server in the router should be turned off. Also, the modem should be set to bridged mode since the router will be handling the connection (actually though, it will work even if the modem is not set to bridged mode).