Q: can you change the apn on a 3g that has iOS 4.2?

I have an iPhone 3g and want to be able to access the internet with my SmartTalk sim card.  I've gone to unlockit.co.nz, but when ever I tap the "Create APN" button, it just sits there and won't load.  Are there settings on the phone that I need to turn off or on before this will work? Thank you!