Where your design is made has everything to do with the materials you have chosen. The first thing to check is that the materials you are wanting to use are currently available in the opening US catalog - which you can check out here:
http://www.ponoko.com/make-and-sell/materials/United-States

That done, here is the process to change your order to be made there:

Go to this page and set your default to the United States:
http://www.ponoko.com/account/preferences

Next, return to your designs section:
http://www.ponoko.com/products

If there is a NZ flag next to your design set instead of an American one, you'll need to click it to edit this.

Then delete the current material applied, using the "delete" link at the bottom right of the material details. You can then choose a new material.

If you've changed your default to the USA on that earlier preferences screen, the materials you'll be choosing from will be from the US hub.

Once you've saved your changes, the product will be set up to be made and shipped from there.