This is a really helpful post on how to side load an eBook to your Kindle, snipped from Kirsta Lakes Author: Go check out her site and buy her books!
“Did your favorite author send you a free book? Are you staring at the .mobi file in your email wondering how to get it onto your Kindle device? Have no fear! Here is the easy quick guide for getting your book onto your Kindle!”
1. Log on to Amazon.com and go to Your Account> Manage Your Content and Devices
2. Go to the Settings Tab
3. Scroll to almost the bottom and you will see these to important areas. First, you will see your Kindle Device’s approved email address. Below that is where you can add an approved email address. If your email address isn’t on the list, add it now. (Don’t add mine! I’m not sending the book to your Kindle- you are!)
4. Email the file (it should be a .mobi or a pdf) to your unique amazon address.
5. Give your Kindle enough time to receive the email! Be sure your Kindle can access the internet. You can also go to Books> menu (the three bars in the upper left corner)>settings> Sync all content to update a little quicker.
6. Enjoy reading your new book!
If you need more help, http://www.iuniverse.com/FAQs/Sideload-Your-Mobi-eBook-File-Onto-Kindle-Fire.aspx has a couple other ways to sideload (like using the USB or the Kindle Fire itself)