|The DD-WRT firmware is not standard with the WRT54G. It may well talk about PPPoE and the fact that it mentions the lack of Modem should be an indication that PPPoE has something to do with a Modem.|
That same page talks (briefly) about using PPPoE to connect to DSL service.
Precisely. You cannot cannot to a DSL service with a router without a Modem. You need a Modem to connect and what comes out of the Modem is not DSL but Ethernet.
This is probably generic software for Routers that do have a Modem built in. The chances are that if there are facilities to configure PPPoE then it wont actually do anything.
Stick with the PPPoE in the Modem where it belongs.