Have you received one of these? Having received one of these in the last few days will show you are on the Electoral Roll and hence eligible to vote. Otherwise you may need to check with South Norfolk Council that you are on the Roll. The contact details are on the card you just looked at, or, see this SNC page for details.
There is still time to register if you wish to vote in the elections on 6 May. You will have to do it before 11:59pm on 19 April.