How to watch an iPhone rental on a Mac?
Do not delete the download or you will lose the rental.

The only method for transferring rentals is between two devices is when you start by renting it from a computer. This is detailed at the end of a quite large knowledge base article under the section worded Can I play my rental on more than one device?.


  If you download a rented movie on your iPhone 4 or later, iPad, iPod
  touch (4th generation or later), or Apple TV: It is not transferable
  to any other device or computer.


You can get help from Apple to accomplish this move if you are not certain if your software is the correct version or are having difficulties with your account. They might issue a one time refund, but this isn't intended to be something you can regularly do. If you have a TV and an Apple TV, your best alternative might be to AirPlay the movie, but on Lion, you will need third party software like AirServer to get video and music from your iOS device (your phone in this case) to play on the mac. Please research this option well (especially the returns policy before buying), since it says it plays videos and it may not work with protected content like a movie rental.