Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
You can download it from App Store. On your iPhone and iPad, open App Store. Search Amazon and you will see the Amazon app in the results. Tap the Get button or cloud button to download. If asked for Apple ID and password, enter your Apple ID and password. If you are referring to the Amazon Kindle app, then search Kindle on App Store.
You don't need the Amazon app on your Mac. You have to browse Amazon website through Safari i.e by going to www.amazon.com
Are you having any difficulty with that ?
If you have any difficulty please let me know and I will continue to help you
Click this link on your iPhone/iPad and it will open Amazon app on your iPhone/iPad --> https://goo.gl/CI368Z
On your Mac, there is no app. You have to go to www.amazon.com on Safari.
If you are referring to the Amazon Kindle reading app, then you can download them from here -> http://www.amazon.com/gp/kindle/pc
If you have any difficulty, please let me know and I will walk you through it.