Just open iTunes on your iPad, log in, then press the movies tab at the bottom of the screen, pick your movie, enter your iTunes password when asked and that's it, you'll start downloading.
Now the ones for rental I maybe being a bit stupid AGAIN but there isnt a lot is there and what there are or that I can see is rather poor. Went for the Love Films App and even they are quite honestly rubbish in choice, am I still doing something wrong?