Babylon on Mac has support for the latest released OSX system, so it should work on your 10.5. After you have completed the purchase of the software you will receive an e-mail containing the serial number which is used to register the application and a link which downloads a .dmg file. In order for the application to be installed correctly you need to double-click it so it can be mounted on the system and a folder will appear on the desktop. Enter that folder and drag the icon with the Babylon name into the
Applications folder. Start the application when it prompts you, enter the serial number you received on your e-mail and you should be good to go.