Read this
http://www.smdepot.net/forum/index.php?topic=28.0it's a 1-2-3 step tut from ZZK on how to make it happen.
basically.....
open "dot halo"
open "coagulation" in "dot halo"
goto "tags"
scroll down to "vehi"
go through the options and find the warthog "flying" check box
check the box, sign the map, and then FTP it back over to your xbox.
for the newer downloaded maps you'll need a different signing program for it. ( this might be why your hog don't fly )
there is a program in our downloads called "samesig" it will sign your new maps.
I havn't modded halo 2 in too long, so maybe there is a better map signer or better halo modding program.
also keep a backup of your "stock" maps ALWAYS
sometimes your cache plays a part of the mods not working, clear out your X Y & Z folders then reboot and see if it works.
good luck
BOOST