You will have to register the domain to the license first so that you could validate it.
Have you used the license to a domain and validate a theme using it? If so, please double check if you are logged in to the same account that you used to register the license in.
If you haven't used the code before, you can simply click on register license and register the license key in there, Once it is registered, it should show up in the list of license on the same page with fields where you can place the domain name.
You can find more information about validating a domain here:
Hope this helps.