I was going crazy trying to find the folder but it was hidden - Here is the fix I found to "UN HIDE" the missing Kindle folder...
Mac OS X 10.7 & OS X 10.8 defaults to hiding the users Library directory, this is probably to keep people from accidentally deleting or damaging files that are necessary for OS X Lion to function properly. That is fine for novice users, but for some of us, we want to be able to access ~/Library/ at will. A visible Library folder was also the default setting in past versions of Mac OS X, here is how to get this back.
Show User ~/Library in OS X Lion & Mountain Lion
Launch Terminal from Spotlight or Launchpad -> Utilities, and enter the following command to show or hide the directory:
chflags nohidden ~/Library/
The users Library folder will immediately become visible again. Reverting this back to the standard Lion setting is simple too:
Hide User ~/Library in OS X Lion (default setting)
This returns to the default setting of hiding the user Library directory:
chflags hidden ~/Library
Changes take effect immediately again, and Library becomes invisible to the user.
Update: temporary one-off solutions are also available for quick access to User Library in Lion
It was there - but I just could see it... [user]/Library/Application Support/Kindle/My Kindle Content
Hope this helps...