Because it is being held at the airport, I would send it here:
Solberg Airport
37 Thor Solberg Road
Whitehouse Station, NJ 08889
Directions to the Festival
The Quick Chek New Jersey Festival of Ballooning takes place every year at
Solberg Airport in Readington, New Jersey
Even though they say its in Readington, NJ, that is the address they have on their webpage.
http://www.balloonfestival.com/info/directions/
Because its an airport though & not really a venue that holds concerts (like say a stadium, arena, etc) you are taking a chance that they will pass on fanmail to her.
Another reason its taking a chance is because a tour schedule also places her here on July 26, the same day:
July 26 Hartford, CT New England Dodge Music Center
If it were me, I would send to this address:
Demi Lovato
c/o Verizon Wireless Music Center
12880 E 146th
Noblesville, IN 46060
This is a venue that is known to pass on fan mail so IMHO you will have a higher success rate.
Address only valid July 23, 2008
Also this same night is Avril Lavigne & The Jonas Brothers, so possible to get all in one night!