Vodacom
Data Bundle deal not working

I saw and advert by Vodacom stating that 500megs prepaid data is now R99, I went to the Rosebank Vodacom shop and an attendant said the deal only works when you buy R105 airtime and convert it to 500megs. Before I bought I specifically told them that I intend to use it on my modem and they said it's the same as normal data bundles. I bought it but now it doesn't work.