Hi,
Thank you for the post.
I think you should set up a PPPOE connection first. To do this, please refer to the following steps:
1. Logon the ISA Server.
2. Click start, and Control Panel, then click Network Connections
3. Click “New Connection Wizard”, select “Connect to Internet”, then “Connect using a broadband connection that requires a user name”
4. Enter the ISA Name, then type the user name and password.
5. Click Finish
After you have created the PPPOE connection, please refer to the following KB to configure ISA Server to this connection.
http://support.microsoft.com/kb/837830/en-us
Regards,
Nick Gu - MSFT