Are you trying to drag the movie to the iPad? That won't work for rented movies.
To transfer a rented movie from iTunes, use the movies pane in iTunes. Rented movies transfer is at the top. You must have an Internet connection to transfer a rented movie. Select the "move" arrow in the direction you wish to transfer, and click the apply button at the bottom right. Rented movies can only exist on one device at a time, they do not sync like other video, but move from iTunes to the iPad.