Previously Afrihost APN's were only able to be provisioned on contract sim cards.
However, singups on pre-paid sim cards are now fully functional.
You can use https://www.afrihost.com/site/product/mobile_data?src=website_nav
to sign up, then select "Exisiting MTN sim card" during the sign up process and you will be prompted for your sim card number.
Don't forget, you can also double your data by activating Afrihost Plus+ :)