Hi,
Thanks for purchasing Pro, I can help out here.
Can you verify that Pro and the Events Calendar are both installed and activated.
Also, if you are using Multisite and both plugins are Network Activated then the license will show in the network settings.
If you have them individually activated the license field only shows on the main site and both plugins should be individually activated there to display it.
We also have this guide about the license key that might have some useful information.
Finding and Inputting Your License Key
Let me know if you have any follow up questions.
Thanks