The IntelliPrint app for Mac requires a minimum of OS X 10.9 (Mavericks).
Once you have downloaded the app, open it. Install the app by dragging the IntelliPrint icon into your Applications folder. Once copied, open the app.
You will be presented with a dialog prompting you to enter your Shopify store's details. Your myshopify domain is the domain given to your store by Shopify. It is the domain you see in your address bar when visiting your store's administration panel, ending with .myshopify.com. Enter your myshopify domain without the leading "https://" and ending at the .com. e.g. intelliprint.myshopify.com
Your App Key is a randomly generated key that you can find to the left of the Settings page of the IntelliPrint dashboard. You will need to enter it exactly as it is shown in the IntelliPrint dashboard.