I rented a movie from iTunes on my laptop, but cannot get it to sync to my iPad. Help!

You don't without Wi-Fi.

Rent a movie. In some areas, you can rent movies. Select the item’s price, then confirm that you want to rent it. You have 30 days to begin watching a rented movie. After you start watching it, you can play it as many times as you want in the allotted time shown onscreen. Once your time is up, the movie is no longer available. Rentals can’t be transferred to another device.

Hi

You can only watch rented movies on the apple device you

Downloaded them on.

Cheers

Brian

Rentals can be transferred from a Computer to an iPad just fine.

The reverse is however not possible. They cannot be transferred from an iOS device to a computer.

About renting movies from the iTunes Store - Apple Support

If you rented the movie on your computer, but want to watch it on an iOS device, connect the devices and sync using USB. When you move a rented movie from your computer, it will disappear from your computer’s iTunes library. You can move the movie between devices as often as you want, but you can only have it on one device at a time.

Once the rental is downloaded, you can watch it on your computer or transfer the rental to a supported iPod, iPhone, iPad, or Apple TV (1st generation):

1. Connect the device and select it in iTunes.
2. Click the Movies button.
3. Select the movie, and then click Move.
4. After all movies have been moved, click Apply in the lower-right corner of iTunes.

You must be connected to the Internet to transfer a rented movie.

Once the movie has been transferred to the device, start playing while on Wifi so it can verify the rental. What I usually do, is connect to the Airport Wifi before getting on the plane, and I start any movie I wish to watch later. Then pause it. Then when on the plane you can watch the rented movie without Wifi.