"Add Value Sets" button in Report Manager Security Workbench

I have a question about value set in Report Manager Security Workbench. Some of our users have "Add Value Sets" button enabled and they can add value sets for security access, some users don't have the "Add value sets" button visible, as a result they cannot add value set for security access. Where is this "Add Value Sets" button enabled or disabled? how do I enable it ?

Thanks