I think you can't get iMessage on ML 10.7.5 correct me if I'm wrong, but I'm pretty sure it is a feature for ML 10.8 and above. So if you can upgrade your Mac, you should get it automatically, but if you can't I guess you're stuck there, sorry.
Unfortunately, OSX Lion was only privy to the beta of the Messages app. You should definitely upgrade to Mavericks to enjoy the function...it is by far one of my favorite functions of my Macbook Pro.