All you've to do, is simply dial *131*200# and then switch-OFF your phone for about 5 seconds and ON it back. That's all!
By so doing, you'll be charged MTN airtime rates for data, and with that, you will be able to use your MTN airtime balance to browse the internet. At long-last, "I can finally use my airtime balance to browse on MTN", I guess, that is want is running through your mind, now. LOL!
NOTE: It's not a trick, "it's official and legit". This is more or less like PayAsYouGo