They provide guided tours with those who want more...

They provide guided tours with those who want more detailed features in the headquarters. The tours are, of course, not for free and can be applied for online but they are totally worthy.

For those who are travelling NYC in not enough budgets, this is not sure but I assume you can just go in the buildings. During the security check at the building right across the HQ, you can say the purpose is for tour. But you would not be able to enter the specially purposed places, but the first floor and the souvenir shop downstairs possibly. Not sure, so please check with others!