Kindle Reader For Mac Os 10.6.8
Find helpful customer reviews and review ratings for AceReader Elite for Mac (OS 10.6.8 - 10.11) at Amazon.com. Read honest and unbiased product reviews from our users. 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.
It sounds as if you're running Firefox, and as of May 4th Mozilla began automatically blocking the Reader plug-in for reading pdf files in web page windows. 
You should have been notified that the plug-in was being blocked. Sean's advice is right - about running in 32-bit mode - but all that seems to have come to a halt on May 4th. Go to Add-ons in the Tool menu and see if you have the 'Adobe Acrobat NPAPI Plug-in, Version 10.1.3' enabled or disabled. I am also curious as to whether you installed the latest Flash player recently as I've had trouble with Reader plug-ins after installing it. The latest version is 11.2.202.235 and you can go to see what version you're running. As an alternate for viewing pdf files in Firefox (or any other Mac browser) you can try 'PDF Browser Plugin 2.4.4' available.
Download Kindle Reader For Mac
Amazon Kindle Reader For Mac
Hope some of this helps - Mozilla and Adobe are apparently working on a fix.