Hi there,
Thanks for writing in! We have done away with the API keys. You can now use the purchase/license code to validate your site directly. If if is not working, please ensure you have the latest version of X and cornerstone installed, then try again.
To check the latest versions, please visit our Changelog
Note: That you may need to delete Cornerstone manually via FTP if the update is not showing up. You can then reinstall it via X Addons > Extensions.
Let us know how that goes. Hope this helps – thanks!