They normally double check the ISIC cards. First when buying the tickets and for the second time when entering. You shouldn't be able buy entrance to Wayna Picchu separately. It has to be bought when buying your ticket to Machu Picchu unless Ministry of Culture has changed their policies during the last 2 months. Anyway, Wayna Picchu tickets has to be bought ASAP, preferably 1-2 weeks before visiting, as they are soldout very often.