Hi

I'm no expert on PPPoE, but can you set the modem to bridge mode and thus pass outside traffic on to the wrt54g and let it handle the username and password for PPPoE connection? Then you would set your wrt54g to dhcp and issue ip address on your local lan.

If no joy, I'd suggest trying the Linksys forums,
Linksysinfo there is probably more specific help there.

Cheers
Rich