i had this setup before, but it became a pain to maintain. used must run your you pppoE dialer and make sure your are connected(virtual connection is active and has an ip). Then install isa and configure everthing. It will work fine until you pppoE connection get disconected or if you reboot.
If the connection is disconnnect. you will have to reconnect and then restart the isa services.
If you reboot, the isa services will start without your connection being active. You will have to stop the services, running the dsl dialer, Make sure you get an ip from your isp, and then restart the is services.
Nick