Go to Spotlight (CMD + SPACE) and type Messages

Messages will open.

Right Click on the icon in the dock and select Options > Keep in Dock.

Click "Not Now" in the sign in windows

In the Menu select Messages > Preferences from the menu at the top left.

Click on Accounts and select Enable Bonjour Instant Messaging.

Messages will then Quit and Reopen.

You should now be able to see who is online.

*iChat has been discontinued, and has been replaced in OS X 10.8 "Mountain Lion" by Messages.