This unlocks the game so it thinks it is the codex edition, I have only tested it on the CD version of the game, so can't say for sure if it'll work on the Steam version.
You will need to have the cracked version for it to work, as the game performs checks on the version you own. (http://fix-my-game.blogspot.com/2011/05/assassins-creed-brotherhood-no.html)
Download this prog:
Mirror 1: http://www.megaupload.com/?d=GIZ7YS0B
Mirror 2: http://www.4shared.com/file/jN2-pCn7/ACB_Codex_Edition_Unlocker.html
Then just double click to install. Simple as that.