If you have a 2degrees microSIM (note: this is only possible from a 2degrees microSIM - we think it's an NZ first, actually) go to Settings.
Tap "Cellular Data", and then "SIM Applications" on the right. At the bottom you should see "Send a text".
It will send an ordinary SMS, so if you want, you can also use it to send an SMS anywhere, not just to 233. Be aware though that we tried every trick we could think of, but the iPad, while it does actually receive the SMS, refuses to do anything useful with it.
If you don't have a 2degrees microSIM, contact customer care to have one sent out to you, and we'll SIM swap so you can keep any credit or bundles you already have.
Hope that helps!