I don't think this will work for 9900/9930 users, based on what I've read elsewhere on this board.
But...if you've got a 9850/9860, go to amazon.com on your phone's browser. Click on Kindle for Blackberrys, then "download now". Will install the Kindle app to your phone. Then register your phone and associate it with your Kindle, and they'll sync up and work great.