First of all, you need to have an US iTunes account in order to access US iTunes Store. Then, you insert the Digital Copy disc into your computer, access US iTunes Store and download the digital copy of the movie into your computer's iTunes. Then, you can watch the movie on your computer or you can transfer it to your iPhone or iPod.
I registered US iTunes account many years ago. So, I don't know if they changed the rules or not. But first of all, you need to have an US address..... Then, you need to have an US credit card or US iTunes Gift card in order to register.