ponimeat

Hope this helps. You can find the full tutorial here.

NOTE: Steam and Rocksmith MUST be installed into the default directory, or RSInjector won't be able to locate and launch Rocksmith. 

Mac: DOWNLOAD
Load Steam first, then unzip the file at any location and launch RSInjector.app as a normal application instead of launching Rocksmith (either directly or from steam).
You can place the executable RSInjector.app wherever you want. It will search for Rocksmith in the normal steam install path (ie ~/Library/Application Support/Steam/SteamApps/common/Rocksmith2014/).
AGAIN: Steam must be running.