How can I get credit for a movie I ordered on iPad but couldn't download because of insuffient data

If you mean that you don't have enough free space available on your iPad then you might be able to (refunds are generally for corrupt downloads, apps not working as described etc), but you can try contacting iTunes Support and see what they say : http://reportaproblem.apple.com

If the 'report a problem' link doesn't work then you can try contacting iTunes support via this page : http://www.apple.com/support/itunes/contact/- click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption