Good afternoon and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
We are sorry to hear about the license key issues you ran into.
I would love to help you with this topic.
What is going on is that before you can input the Events Calendar Pro’s license key, you need to first download, install and activate the Events Calendar Pro add-on (on top of the Events Calendar).
This will create an additional license key tab for the Events Calendar Pro.
You might want to follow along this guide to make this a walk in the park: http://theeventscalendar.com/knowledgebase/new-user-primer-the-events-calendar-and-events-calendar-pro/
Let me know how that goes.
Best regards,
Geoff B.