Hello There,
Thanks for writing in! If you are still using an older version of X, you can use the license code as a replacement for the API key. Please do the following:
1] Please login to your account and assign the license. You need to assign the license to your site using this url: https://www.your-domain.com/. You have to make sure that url you just assign is the same as the one you have in your settings, Settings > General > Site Address
2] Go back to X Addons > Product Validation again, and insert the license code in the API key field.
Hope this helps.