In order to download the correct version of your purchased software, you need to log into the Toon Boom Animation website at www.toonboom.com. When you purchased your product, you were made to create an account with a username and password. This is the username and password that you need to log onto the website.
Once you have logged into the website, you can see all the Toon Boom products that you purchased under the My Downloads tab. On the My Products tab, beside the product name, you will see a Download link. Click on that link to bring up the Licensed Product Download page. Once there, you will have the option to download any available platform version.
The original license key that you were sent can be used to activate both the PC and the Mac versions of the product.