This has been out a while, but couldn't find any post relating to it on here.
So for those who don't know......
Go to xbox live follow the link, download the halo waypoint software
Just for downloading it and running it for the first time your be given the Halo Waypoint Monitor, Go to My Xbox, open your profile and select Customise Avatar.change my style, then go to props and it should be there, select it then save and exit
There are other awards that give you free avatar items with the halo waypoint software
With halo 3 odst
Unlock Halo 3: ODST Vidmaster Endure Achievement
Unlock Halo 3: ODST’s Campaign Achievement
Unlock Halo 3: ODST’s Dark Times Achievements
Unlock Halo 3: ODST’s Good Samaritan Achievement
Unlock Halo 3: ODST’s Tourist Achievement
With halo Reach
Kat's Helmet: Avenged a teammate's death in Matchmaking.
Jun's Helmet: Kill 100 enemies in a row without dying in either Campaign or Firefight.
Carter's Helmet: Cleared a Campaign mission on Legendary without dying.
Emile's Helmet: Earn a Bulltrue medal in multiplayer