I believe you have to register a profile under
http://nbaofficials.com/ and type in your officiating resume and basic info. Then you can get invited. I got an invite last year..and I believe it was $500 (not including lodging/flight).
This is last year's info:
http://www.nba.com/dleague/predraftcamp/index08.html
I wasn't able to go...but may want to give it a shot...when I think I'm ready.
-KG