MTN South Africa Pay as you go SIM Card

I bought an MTN pay as you go card for South Africa. I cannot see how to
activate it in the instructions. Last time I bought an overseas card it
worked immediately in the UK so you could see it works. Does anyone know
the procedure for getting started as it does not log onto a UK network. Can
I test it in the UK or do I have to do something when arriving in South
Africa

Thanks

Graham