I was able to install Mountain Lion on my Mac with no issues. I have a mid 2010 macbook pro running an i5. When I launched the Messages app, all my accounts where loaded in there. The issue that I am having is that my iMessages account will not show me on line, even though the account has been enabled for use. None of my conversations that I am having on my iPhone will load automatically or show in the Messages app. I have checked to make sure that the iCloud account on my phone and the iCloud account on my mac are the same. My calendar and reminders sync just fine between the two. I have spent over a hour with apple care support and they cant figure out why its not working. Any body out here have any idea? I have tried a fresh install twice of the OS. The accounts are all entered and logged in correctly. I was able to send one message from the Messages app that went through to someone and showed on their phone that the message was sent from my email. The rest of the coversation did not carry over. Some additional information: When I click the messages menu and scroll down to accounts, iMessage is checked. When I click on the status menu, every thing is in light gray and its showing off line, and I cannot change the status. On the bottom of the messages app interface, it is showing my yahoo account as on line, my iMessage account is not listed there. My internet connection is working fine and so are all other OS functions and iCloud features. My iMessage account has been enabled under the preferences > accounts. I have tried logging out and back in. No conversations are listed in the Messaging app at all.