Thats a pretty old post Mook. Kindle for iPad syncs with iTunes. Kindle app for Mac doesnt it's a stand alone application. From memory you can add to the library from one of the menus. Possibly dragging a mobi file onto it would add the book as well.
I haven't used it for ages, I mainly use epubs. If you have an account amazon gives you an email you can send books to that will load into the cloud which can then be downloaded to the Mac app or reading device.
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