You shouldn't post the same post in two different threads.
That said, download and utilize the Send To Kindle Mac app. The books (assuming they are in a compatible format and are not DRMd) will then appear on the Kindle when you sync to Amazon.
http://www.amazon.com/gp/sendtokindle/mac
As for your mounting problem, it could be a conflict on the Mac, or a problem with the USB cable.