Disappointed in Service

I had to buy data bundles for my computer USB vodafone stick and despite the fact that i am registered with Vodacom, and that my phone indicates that i may buy additional bundles, i was told that it was only possible by visiting the branch. <br> <br> The essence of my inquiry is why do Vodacom offer this service on my cellphone that i may buy bundles and when activating this service, it tells me that i am not a registered user.<br> <br> So how do i proceed in purchasing data airtime for this computer USB stick??