You should've recieved your key in an email, separate from the one with download links. Due to email sorting the email could've been moved to a Promotions tab or Spam. Sometimes, when a higher amount of orders has to be processed, the email could be delayed.
You can find all your license verification keys at www.plugins.oversampled.us.
All you have to do is:
1. Log into your account (or create it) with the email you have used upon purchase.
(Important note: plugins.oversampled.us and oversampled.us do not share user accounts)
2. Click "Your Plugins" on top of the website.