Hello John,
Validating the theme installed in your localhost is the same as validating a live or staging site. Simply go to your APEX account and manage your licenses (https://theme.co/apex/licenses). Make sure that the license is assigned to your localhost site url which can be found in Settings > General.
To validate X or Pro properly, 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: {your localhost site url}
. 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] In your localhost, go back to X > Validation again, and insert the license code.
And please review the changes that we made on how you are going to validated your X theme because we made a big change in the process. Please check it out here: https://theme.co/apex/forum/t/setup-product-validation/55
Hope this helps.