If you plan on pitching your tent anywhere outside of the designated camping areas in the ENP you will need to get a Special Use Permit from the Park. These don't come fast or cheap: $100.00 plus two weeks processing. Follow the link to get contact information and download a permit application (PDF format, requires Adobe Acrobat Reader):
Disclaimer: I have never actually gotten one of these myself (I like to sleep in my boat!), so I don't know the exact procedure or exactly what circumstances require one, so you will want to call the contact number given on the ENP site. I will be investigating this further and will post whatever information I find out.