You can download KindleGen tool via http://www.amazon.com/gp/feature.html?ie=UTF8&docId=1000234621
How to use
1. Open Terminal program (on Mac) / Command Prompt program (on PC).
2. Type KindleGen mybook.epub in command line.
3. Wait until it completes.
4. The file mybook.mobi is generated. It can be read by Kindle.