Chatting in Messages on OS X
Messages on OS X 10.8 and later has combined the functionality of the old iChat app on your Mac and iMessages on your iPhone. This is causing some confusion over which accounts can be used for screen-sharing & chatting, etc.
Your iMessages account allows messaging other iMessages accounts -- this is the one integrated into your iOS devices, and allows sending and receiving text messages to other iOS users. This account does not allow screen-sharing, audio, or video chatting.
A separate account needs to be added to Messages for screen-sharing, audio, and video chatting. The @icloud.com or @me.com email address that Apple issued with your iCloud account can be used for chatting through the AIM servers (Aol Instant Messenger) by setting it up as an Aol chat account. We recommend using this because you already have one! Go to Messages >> Preferences >> Accounts and add a new Aol account. Configure this with your Apple email and password.
Alternatives are to use a GoogleTalk account with any Google account (@gmail), a separate Jabber account (which includes GoogleTalk and Facebook), or an Aol chat account (AIM). Yahoo Buddies does not support screen-sharing, video, or audio chatting.
Once one of the supported chat accounts has been added to Messages, go to the Window menu, and select Buddies to open the chat contact list (a shortcut to show this is "command+1"). From the Buddy List you can add new "buddies" and start any of the chat options via buttons at the bottom of the Buddy List window.
1. Lets say you want to use your Apple email (such as an iCloud issued @me.com address) as the Messages account.
2. Go to the Messages >> Preferences >> Accounts and click the "+" button at the bottom of the accounts list.
3. Choose Aol from the menu.
4. Add the @me.com address in full.
5. Click Done. You will now have an AIM Account in the Accounts list.
6. Select the Window menu, and you will now have a Buddies option (also, shortcut "command+1").
7. This will show you the Buddy List window.
8. Add contacts ("buddies") to your list via the "+" button at the bottom of this window.
9. Finally, in the Buddies window, set this account to "Available."