After purchasing, you would have received several emails, one of which would have been an email with a specific link inside it for you to click that would then take you to a page to finish officially setting up your account.
Sometimes the invitation gets filtered, so can you try doing a search for “Atticus”, making sure it also checks your spam, promotional and junk folders? It should have the subject line, “Create Your Atticus Account.”
Once you have your account set up, you can access it in a browser by visiting app.atticus.io or you can install it as an app on your computer. The browser and app version offer the same functionality, including the ability to write while offline. You do need to be online to import and export files, however, and you'll have to make sure you're logged into Atticus and have the program open in your browser before you lose your internet connection.