Your license key will be automatically emailed to the email address you used during checkout. You'll also receive a separate order confirmation email from Cerberus via FastSpring, the reseller who runs our online store.
These emails should appear after just a few minutes -- if you don't see them, please check your spam folder (unfortunately things can still end up there!). If you're still missing the key, send a note to firstname.lastname@example.org. We'll be happy to send you a copy.