Amazon produces multiple beta test builds of new Kindle for PC/Mac versions and delivers them to a small group for testing. Who gets them and when appears to be random. They can have serious bugs and are not always the final build that eventually gets released for that version.
If you want to try it at your own risk, you can download the most recent Kindle for Mac 1.26 build (as of today) here:
https://s3.amazonaws.com/kindleforma...rMac-55087.dmg