PC: Copy file: "_p.psarc" to "<Rocksmith2014 folder>\dlc";
Mac: Copy file: "_m.psarc" to "~/Library/Application Support/Steam/SteamApps/common/Rocksmith2014/dlc/" , you might need to use "Command-Shift-G" to go inside ~/Library since it's hidden by default in the finder;
XBox 360: Place files: "_xbox" to "Content>0000000000000000>555308C0>00000002";
PS3: Install files: "_ps3.psarc.edat" to "/dev_hdd0/game/BLUS31182/USRDIR/DLC", if the folder DLC does not exist, just create it.
Note: For PC and MAC need an extra step:
Note2 : For PC and MAC you need to change App ID of custom song package with RS Toolkit to App ID of an official DLC that you had bought (Cherub Rock is most used on customs)
PC alternative 1:
DOWNLOAD Copy to Rocksmith 2014 root folder;
Run the game by normal Rocksmith2014 executable app (will popup a console debug window, it's normal, don't worry)
Thanks to Zephyrix for this patch.
If first one doesn't work, use this updated version:
Updated version -> DOWNLOAD
PC alternative 2:
DOWNLOAD Copy to Rocksmith 2014 root folder;
Run the game by this executable (Rocksmith_custom.exe)
Thanks to Alex360 for this patch.

Mac:
DOWNLOAD To downlaod click file and at the bottom download.
Just unzip the file 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/).
Steam must be running.
Thanks to Xavatar (Power Slinky) for this patch.

JUST PLAY IT !
Delete my account please.