Is casual data (like the $1 up to 10mb) possible on iPhone?

After asking what my plan was Voda told me yesterday via chat on the Vodafone store it should be fine and I should try it out. My plan is the ancient Get 200 from a million years ago, but they seem to think that is ok.

Tried it, doesn't work - "You are not subscribed to a cellular data service".

I get casual data fine when using a voda pre-pay sim (or telecom prepay) just not on my voda plan.

Anyone know what the story is for sure? and is it some configuration I can change?

Running 4.2.1 (no jail break) on a 3G purchased new from Vodafone NZ

[EDIT: purchased with no plan at all just used the sim from my previous voda phone]

[EDIT: Forgot to mention: If I recall corrrectly, I used http://www.unlockit.co.nz years ago to change something to allow me to get prepay internet]

Lastly, is tethering likely to work for me? Never tried it.