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If you had iChat installed, after you've finished installing Messages Beta any configured messaging services--such as AIM, shown here--will auto start with Messages' configuration program.
To get started with Messages Beta, you must first enter your existing Apple ID and password. If you don't have an Apple ID, Messages gives you the opportunity to create one. After entering your Apple ID and password, click the Sign In button.
Messages gives you the opportunity to check up on the status of all your messages. If you want a read receipt sent for every message you write, check off the Send Read Receipts box. If you change your mind later, you can always change this setting in the Preferences dialog box.
That's it! Messages is ready to go. Click the Done button to launch the main application window.