interface (whatever)
nameif outside
pppoe client vpdn group CHN
ip address pppoe
!
mtu outside 1492
!
vpdn group CHN request dialout pppoe
vpdn group CHN localname (your username)
vpdn group CHN ppp authentication pap (or whatever you need)
vpdn username (your username) (your password)
Referência: Cisco" PIX / ASA: exemplo de configuração de cliente PPPoE "docs