One your freshly installed Mountain Lion, you will see a blue bubble icon of iMessage in the dock. If you already have an iMessage account or already setup your iCloud login than you can skip to the next step.
This version of iMessage on Mountain Lion also supports the other IM clients such as Gmail, Yahoo, AOL and Jabber. However, you can’t add them simply; you can follow these steps to add a Non-Apple IM.If you haven’t open the iMessage, open it and from the top bar Click on “Message” and then “Preferences”. It will open the settings related to iMessage.