eleanor has yet to fill out a bio.
Whew...a registered letter? That's tough. I know you can send an email to the league office at http://www.nba.com/email_us/contact_us.html
I would have to guess he gets mail at NBA headquarters. So I'd send it to:
NBA Corporate Office & Headquarters
645 Fifth Avenue New York NY 10022
Thanks for your fan-dom, my friend...