If you don't have this installed already, you've missed your chance.
If you haven’t already installed the Messages application on your Mac, you will now have to wait until OS X Mountain Lion is released next month to get your hands on it. The Messages for Mac beta, which was released back in February after Apple’s initial Mountain Lion preview, has now been pulled by the Cupertino company.
It’s unclear when or indeed why Apple removed the Messages beta from its site. I had an issue with the application late last night, and I decided to uninstall the application and then install it again to see if this solved the problem. However, the Messages for Mac beta page has been removed from Apple’s site, and when visiting the URL, you are redirected to the OS X page.
There’s still a Messages for Mac link at the bottom of Apple’s Downloads page — as you can see below — but again, it just redirects to the Mac OS X page, where a Messages beta cannot be found.
It’s likely that Apple is now happy with how beta testing has gone, and that it has already prepared a final release that will launch alongside Mountain Lion in July.
If you already have Messages install on your machine, it’s probably best to hold onto it for a few weeks.