Carbon is used to remove impurities from the water and so will remove the medication, obviously reducing it's effectiveness.
While I have been lucky enough not to encounter ich if my fish were to get it I would use the 'salt' method for a couple of reasons, I keep loaches which probably doesn't apply to you, and I don't like to add chemicals to the aquarium when there is a natural alternative.
I'm sorry but I don't know the specifics of the 'salt' ich treatment but I think that it is to basically add salt up to 5g/Gallon(not sure exactly) to stop the parasite multiplying, and raise the water temp to quicken the parasites lif cycle.
Also remember that once the ich disappears you should still continue your chosen treatment, though maybe at a lower dosage as the parasite is more than likely still present in your tank.