Microsoft Office (like any other applications) has its own installer which is used to deploy the package on a system (Mac or Windows). If you have a CD with the package, insert the CD, double click the icon on the desktop and follow the on-screen instructions. If you already have the setup file, double click it and the Mac will mount the archive and a new folder will appear on your desktop. Execute the installer from the folder and follow the same instructions. You will need to have your CD-Key near because the installer will prompt you to type the CD-Key.