It sounds like you installed it wrong. On a Mac, you have to 'right-click' the app and select 'show pakage content', then navigate the folder that opens up, untill you find the folder named "game".. Now leave that window open, and open a NEW window where you have the
mod, extract that, and you'll have a folder named "game".. Hold 'Alt' and drag&drop the 'game' folder from the
mod, into the window you got from the 'right-click app-->Navigate folder till you find 'Game', If you did it right, it should ask you if you want to 'overwrite' or 'replace' or 'merge'..
Select 'Merge'..! That's the key to make it (any mod) work on Mac.