The easiest way to download your novella/book is to follow the instructions for your device after clicking  'Get My Book' - for most devices it will suggest downloading the Book Funnel App. I know people are a little wary of apps but Book Funnel are an internationally renowned organisation that work with thousands of authors. You can use the email/Send-to-Kindle option though this is less convenient. If you have one of the touchscreen Kindles, the web browser option is also very easy to use.
Downloading your file onto the computer and then sending it on to your devices is by far the most difficult process - life is too short unless you are an expert (unlike me).

Book Funnel App

Send-to-Kindle