Hello,
Sorry to hear you are having issues. Try this, uninstall CursorFX, reboot immediately (important), download latest version available in your account product download page: https://store.stardock.com/myaccount/products. Install it, reboot immediately again. Try apply one theme, it should ask for your Product key, please enter your email address and your product key. That should activate your software.

Thank you,
Basj,
Stardock Community Assistant.