What do I do if my in app purchase won't download?

If it hasn't downloaded then try the 'report a problem' page to contact iTunes Support : 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