You're correct, Advantage doesn't treat tapeworm. Ask your vet to give you some Drontal Plus (you shouldn't have to bring your dog in again if the vet has already confirmed it's tapeworm...it'll save you another consultation fee). Drontal is usually administered one day...then repeated for one day two weeks later. Please do not give your dog anything you can find at a pet store...it can be harmful.
"Education is a progressive discovery of our own ignorance." -Will Durant