Add/Switch iMessage account

You cannot add an iMessage account. You just sign in or sign out. You can only be signed in to one imessage account at a time. You CAN, however, add a messages account. iMessage and messages accounts are differemt, but both used within the Messages application.

Example:

My iCloud apple ID is a gmail account. From within the gmail apple id, I have created a me.com email address. I sign in to iMessage with the me.com account but I enter the gmail.com account under the "You can be reached for messages at:" section (or vice versa). I can do this because both email addresses belong to the same apple id account.

I also have a second gmail account that is also an apple id. After setting things up as per the previous paragraph, I went in to Mail, Contacts and Calendars in the system preferenes and added a Messages account via "Other". You can also accomplish this by pressing the "+" symbol in the very bottom left of the accounts section of the Messages preferences.

I'm having exactly the same problem. Can't add my secondary email address/Apple ID, as I get the message about it already being in use. On my iOS devices, I'm able to do this just fine, so not sure why I can't accomplish this on Mountain Lion.

So when I click on the "+" sign at the very bottom left, OR when I go into Mail, Contacts and Calendars under System Preferences and select "add other account", I'm presented with a window that asks me to select what type of account I want. The choices I'm given are: AIM, Yahoo, Google Talk & Jabber. Shouldn't I also see iMessage??

From that Mail, Contacts & Calendars setup window, I did try adding my other iCloud account - but then it asks me to choose what to share in iCloud - mail, contacts, calendars & reminders, notes. Not messages....

I'm really stuck!

Thanks SO much! I finally got it. I was getting hung up on the sign-out step.. I couldn't figure out how to do that, but it I finally noticed the sign out button in the Account Details screen. It didn't dawn on me that I needed to sign-out of the current account I was logged into, before I could log into the one I actually wanted. I usually have no problems wrapping my head around Apple products, but this one had me really scratching my head.

Thanks again.. that was a huge help! 🙂

Hmmm I think you're confused. If you select your imessage account in the left column, there are 2 "+" symbols. The one you want is in the very bottom left of the messages preferences. The inner most "+" is for adding alternate email addresses that are associated with with apple id you've signed in to imessage with. To see a list of the associated email addresses, sign in to http://appleid.apple.com with the apple id you're currently signed in to imessage with.

Short answer: to make it easy, go into Mail, Contacts and Calendars under System Preferences and add a messages account there (scroll down and choose "add other account" in the bottom rght)

Good luck!

No, you're already signed in to iMessage and as per my previous posts, you can only sign in to iMessage once. You can, however, also sign in to other chat services at the same time (such as google talk or AIM etc.). You cannot add or remove an iMessage account. You can only sign in or sign out. And you can only be signed in to one iMessage account at a time. IF you have other email addresses ASSOCIATED with the account you've signed in to iMessage with (these are viewable by signing in to appleid.apple.com), you CAN add them to the "You can be reached for messages at:" section with the inner "+" symbol. BUT if you've already associated an email address with a DIFFERENT apple id or if it is an apple ID itself, then you will NOT be able to add it with the inner "+" symbol. I know it's confusing, but read through this thread a few times and you will understand. Cheers!

Edit: I suppose the thing to take away is that:

Messages is the App you're using

iMessage is a service you sign in to with an apple id

You can also sign in to the Messages app with chat services other than iMessage (such as your google talk account, using your gmail account and gmail password)