After many attempts I found out that the issue I was facing was because I was pasting the custom dlc files into the folder ~/Library/Application Support/Steam/SteamApps/common/Rocksmith2014 rather than ~/Library/Application Support/Steam/SteamApps/common/Rocksmith2014/dlc
after copying the files to "dlc" folder it started to work for me.
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After many attempts I found out that the issue I was facing was because I was pasting the custom dlc files into the folder ~/Library/Application Support/Steam/SteamApps/common/Rocksmith2014 rather than ~/Library/Application Support/Steam/SteamApps/common/Rocksmith2014/dlc
after copying the files to "dlc" folder it started to work for me.