No, you cannot carry in the visitor's center.
The law does indeed permit a person to carry in National Parks in States where carrying in such a manner is also lawful. The law however makes an exemption for NPS buildings where a sign is clearly posted. So carry with confidence on the grounds of the park, but do not carry in any building where it is forbidden with sign-age posted.
CAVEAT: I am not a lawyer and I am definitely not your lawyer. Read the law for yourself and come to your own conclusions.
http://home.nps.gov/news/release.htm?id=962