My iMac has v10.7.3 I cannot find "Messages" on this computer

My early 2008 iMac has Version 10.7.3 but does not have "Messages" on it. It does have iChat. My early 2008 MacBook Pro has the same version of Lion but it does have "Messages" Where can I go to download "Messages" for my iMac. It is infinitely more useful than iChat