Does your DSL modem have a router built in? You didn't mention it, but it sounds like it does, otherwise you should need to run pppoe software and the connection would not be there when you boot Knoppix.

If you are not using a router I strongly suggest that you buy an inexpensive one and use it.

The answer to your question (assuming a router to take care of pppoe for you is somewhere) should be to run netcardconfig and answer the dhcp question that it asks in the affarmative.