google -> msn mac
1º link -> procurar instruções de como instalar
=
5. If the file you downloaded does not expand automatically, on your hard disk, double-click the file that you downloaded in step 4.
6. To copy Messenger for Mac to your hard drive, drag the Microsoft Messenger icon to the location where you would like to save it. Then, double-click the Microsoft Messenger icon to start the setup program.
7. Click Next.
8. Read the Windows Live Messenger Service Terms of Use and, if you accept these terms, click Next, and then click Accept.
9. Follow the instructions on the screen.
dois segundos a procurar e encontrei a resposta. enfim...