I have bought Talking Ben app on my iPhone. I also have an iPad, how can I transfer this purchase onto my iPad?
I already tried using my iTunes. But i dont know how to do it. Also both my iPad and and iPhone use the same Apple ID.
There are (at least) two ways to accomplish that
directly on the iPad
The iPad option only works if you are logged into the App Store with the same Apple ID on both devices, the iTunes option also works with different IDs.
As pointed out in the comment certain applications provide a specific iPad version which must be bought separately. You can still install the iPhone version in the iPad as well but will have to accept the usual restrictions concerning screen size unless the app was specifically coded to work both on iPad and iPhone.