After you purchase Storyist from the Storyist website, your license information is presented on the checkout page, and also sent to the email address you provide at checkout.
To install it:
Launch Storyist. When the Trial dialog appears, click the Enter License Code button.
Enter your license name and code exactly as it appears in the license email, caps and all.
If you need to install Storyist, you can download it from http://update.storyist.com/downloads/Storyist.dmg.
If this doesn't work for you, take a look through the troubleshooting guide at http://storyist.com/support/howto/install-your-license/.
-Steve