I did not find this. Credit goes to Tornsoul.
What you do, is you put said pre order files into horizon and inject them into your save device, as id swaps with dlc are not possible, they are all 0's. once they are on your device, play a match online. now log out, go onto your computer and log onto xbox.com and purchase a weapon skin set for GOW3. play another match online in GOW3. after the match the preorder bonuses should be unlocked. I hope this clarifies things and that it is successful for you!
The Green liquid metal skin: http://www.mediafire.com/?fhfjmgff6fqtlxn Credit goes to Chronic
The collection(or Epic) skins: http://www.megaupload.com/?d=YLYKFHXQ Credit goes to dean_64uk
Good luck and enjoy!
EDIT: The preorder DLC is corrupt. just use the Skins. Removed Preorder DLC link.