I purchased a movie rental on my iphone and have not been able to download it due to lack of space on the phone. I would like to instead finish the downloading on my macbook but have not been able to do so. How do I download an existing purchase?

Rentals are a one-time only download from the store. If you can't free up enough space on your phone for it then you can try the 'report a problem' link to contact iTunes Support and see if they grant you a redownload for your Mac : 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