First off, you need to have a valid product key. SO3 keys are no longer available, but you might find an old version with a valid key for purchase on an auction type website. Just make sure the seller verifies the key is un-used or the box is still sealed. Login to https://my.presonus.com/, under your products click on the version you own. On the download page, you will see a Download Installer - Mac button if you are currently running a macOS computer. If not currently on a Mac, click the Show All Systems button and you will see download buttons for all available OS's.