I rented a movie using iTunes on my iPhone, where does it go so I can watch it?

If you have iOS5 or newer, it will be in the videos app.

The Videos App should be on the first screen somewhere, its a black and blue icon and says "Videos", check inside your folders if you cant find it. Once you find it, tap it, if the title "My sister's keeper" is not there, try tapping the "store" button on the top left of that page. Tap purchased and see if you can find the film, if it doesn't say downloaded near it, you can tap download which should be near the film title. Also, you can try pressing "videos" then "store" then "more" (on bottom right), then "downloads", it should be there and try download it back to your iphone. Or maybe the download has not fully completed yet, worst comes to worst, find the film on itunes on your computer, download it (it will recognise that you've rented it) and transfer to your iphone.

If u downloaded it from the computer it's in the iTunes library from your iPhone u find it in your videos